Move Split Common VPATH addition (#4716)

Specifically, only add the split_common folder to COMMON_VPATH if the feature is eanbled, to prevent issues with compilation

common.mk

@@ -21,5 +21,4 @@ COMMON_VPATH += $(QUANTUM_PATH)/keymap_extras
 COMMON_VPATH += $(QUANTUM_PATH)/audio
 COMMON_VPATH += $(QUANTUM_PATH)/process_keycode
 COMMON_VPATH += $(QUANTUM_PATH)/api
-COMMON_VPATH += $(QUANTUM_PATH)/split_common
 COMMON_VPATH += $(DRIVER_PATH)

common_features.mk

@@ -268,4 +268,5 @@ ifeq ($(strip $(SPLIT_KEYBOARD)), yes)
                 $(QUANTUM_DIR)/split_common/split_util.c
     QUANTUM_LIB_SRC += $(QUANTUM_DIR)/split_common/i2c.c
     QUANTUM_LIB_SRC += $(QUANTUM_DIR)/split_common/serial.c
+    COMMON_VPATH += $(QUANTUM_PATH)/split_common
 endif