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.h

@@ -1,5 +1,10 @@
 #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"
 
@@ -14,7 +19,3 @@ void split_keyboard_setup(void);
 bool is_helix_master(void);
 
 void matrix_master_OLED_init (void);
-
-// stubs as this is handled by legacy code
-static inline void split_pre_init(void){}
-static inline void split_post_init(void){}

keyboards/helix/rev2/local_features.mk

@@ -52,9 +52,10 @@ ifneq ($(strip $(SPLIT_KEYBOARD)), yes)
 
   CUSTOM_MATRIX = yes
 
-  SRC += rev2/matrix.c
+  SRC += rev2/custom/matrix.c
-  SRC += rev2/split_util.c
+  SRC += rev2/custom/split_util.c
-  SRC += rev2/split_scomm.c
+  SRC += rev2/custom/split_scomm.c
+  KEYBOARD_PATHS += $(HELIX_TOP_DIR)/rev2/custom
 endif
 
 ########