Fix helix/rev2 not working properly when using split_common. (#16512)

When helix/rev2 was using split_common, it didn't work properly and I couldn't type on the right hand side.

The cause is that the following code, added in 0.16.0 to `quantum/keyboard.c`, does not include `quantum/split_common/split_util.h` but instead includes `keyboards/helix/rev2/split_util.h`. Therefore, `split_pre_init()/split_post_init()` in `quantum/split_common/split_util.c` was not called.

```c
  #ifdef SPLIT_KEYBOARD
  #    include "split_util.h"
  #endif
```

keyboards/helix/rev2/custom/split_util.c

@@ -0,0 +1,109 @@
+#include <avr/io.h>
+#include <avr/wdt.h>
+#include <avr/power.h>
+#include <avr/interrupt.h>
+#include <util/delay.h>
+#include <avr/eeprom.h>
+#include "split_util.h"
+#include "matrix.h"
+#include "keyboard.h"
+#include "wait.h"
+
+#ifdef USE_MATRIX_I2C
+#  include "i2c.h"
+#else
+#  include "split_scomm.h"
+#endif
+
+#ifdef EE_HANDS
+#    include "eeconfig.h"
+#endif
+
+#ifndef SPLIT_USB_TIMEOUT
+#    define SPLIT_USB_TIMEOUT 2000
+#endif
+
+#ifndef SPLIT_USB_TIMEOUT_POLL
+#    define SPLIT_USB_TIMEOUT_POLL 10
+#endif
+
+volatile bool isLeftHand = true;
+
+bool waitForUsb(void) {
+    for (uint8_t i = 0; i < (SPLIT_USB_TIMEOUT / SPLIT_USB_TIMEOUT_POLL); i++) {
+        // This will return true if a USB connection has been established
+        if (UDADDR & _BV(ADDEN)) {
+            return true;
+        }
+        wait_ms(SPLIT_USB_TIMEOUT_POLL);
+    }
+
+    // Avoid NO_USB_STARTUP_CHECK - Disable USB as the previous checks seem to enable it somehow
+    (USBCON &= ~(_BV(USBE) | _BV(OTGPADE)));
+
+    return false;
+}
+
+
+bool is_keyboard_left(void) {
+#if defined(SPLIT_HAND_PIN)
+    // Test pin SPLIT_HAND_PIN for High/Low, if low it's right hand
+    setPinInput(SPLIT_HAND_PIN);
+    return readPin(SPLIT_HAND_PIN);
+#elif defined(EE_HANDS)
+    return eeconfig_read_handedness();
+#elif defined(MASTER_RIGHT)
+    return !is_helix_master();
+#endif
+
+    return is_helix_master();
+}
+
+bool is_helix_master(void) {
+    static enum { UNKNOWN, MASTER, SLAVE } usbstate = UNKNOWN;
+
+    // only check once, as this is called often
+    if (usbstate == UNKNOWN) {
+#if defined(SPLIT_USB_DETECT)
+        usbstate = waitForUsb() ? MASTER : SLAVE;
+#elif defined(__AVR__)
+        USBCON |= (1 << OTGPADE);  // enables VBUS pad
+        wait_us(5);
+
+        usbstate = (USBSTA & (1 << VBUS)) ? MASTER : SLAVE;  // checks state of VBUS
+#else
+        usbstate = MASTER;
+#endif
+    }
+
+    return (usbstate == MASTER);
+}
+
+static void keyboard_master_setup(void) {
+
+#ifdef USE_MATRIX_I2C
+    i2c_master_init();
+#else
+    serial_master_init();
+#endif
+}
+
+static void keyboard_slave_setup(void) {
+
+#ifdef USE_MATRIX_I2C
+    i2c_slave_init(SLAVE_I2C_ADDRESS);
+#else
+    serial_slave_init();
+#endif
+}
+
+void split_keyboard_setup(void) {
+   isLeftHand = is_keyboard_left();
+
+   if (is_helix_master()) {
+      keyboard_master_setup();
+   } else {
+      keyboard_slave_setup();
+   }
+   sei();
+}