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
```

1 files changed, 21 insertions, 0 deletions

diff --git a/keyboards/helix/rev2/custom/split_util.h b/keyboards/helix/rev2/custom/split_util.h
new file mode 100644
index 0000000000..3b1d4249f5
--- /dev/null
+++ b/keyboards/helix/rev2/custom/split_util.h
@@ -0,0 +1,21 @@
+#pragma once
+
+#ifdef SPLIT_KEYBOARD
+#   error This is helix local split_util.h.
+#   error This header file is used only when SPLIT_KEYBOARD=no.
+#endif
+
+#include <stdbool.h>
+#include "eeconfig.h"
+
+#define SLAVE_I2C_ADDRESS           0x32
+
+extern volatile bool isLeftHand;
+
+// slave version of matix scan, defined in matrix.c
+void matrix_slave_scan(void);
+
+void split_keyboard_setup(void);
+bool is_helix_master(void);
+
+void matrix_master_OLED_init (void);