[Bug] Fix compilation error when Split Watchdog enabled (#21543)

quantum/split_common/split_util.c

@@ -31,6 +31,10 @@
 #    include "rgblight.h"
 #endif
 
+#if defined(SPLIT_WATCHDOG_ENABLE)
+#    include "bootloader.h"
+#endif
+
 #ifndef SPLIT_USB_TIMEOUT
 #    define SPLIT_USB_TIMEOUT 2000
 #endif