Add pointer(?) to disable_JTAG to prevent compiler errors (#4310)

quantum/split_common/split_util.c

@@ -92,6 +92,7 @@ void split_keyboard_setup(void) {
    sei();
 }
 
+void disable_JTAG(void);
 void keyboard_slave_loop(void) {
    // Disable JTAG since we skip calling keyboard_init() on the slave side
    // Future fix will possible call keyboard_init() on the slave to remove this need