process_combo: restore wait.h header (#21514)

Required for `wait_ms()` function

quantum/process_keycode/process_combo.c

@@ -19,6 +19,7 @@
 #include "process_auto_shift.h"
 #include "caps_word.h"
 #include "timer.h"
+#include "wait.h"
 #include "keyboard.h"
 #include "keymap_common.h"
 #include "action_layer.h"