fix(split): Conditionally build all split code (#2884)

Don't add the split CMake subdirectory unless the feature is enabled.

Author: petejohanson

app/CMakeLists.txt

@@ -96,7 +96,7 @@ target_sources_ifdef(CONFIG_ZMK_BATTERY_REPORTING app PRIVATE src/battery.c)
 target_sources_ifdef(CONFIG_ZMK_HID_INDICATORS app PRIVATE src/events/hid_indicators_changed.c)
 
 target_sources_ifdef(CONFIG_ZMK_SPLIT app PRIVATE src/events/split_peripheral_status_changed.c)
-add_subdirectory(src/split)
+add_subdirectory_ifdef(CONFIG_ZMK_SPLIT src/split)
 
 target_sources_ifdef(CONFIG_USB_DEVICE_STACK app PRIVATE src/usb.c)
 target_sources_ifdef(CONFIG_ZMK_USB app PRIVATE src/usb_hid.c)