Overhaul scaninc to work recursively

This also fixes the bug where scaninc would ignore #include lines
in assembly files.

src/main_menu.c

@@ -1,3 +1,4 @@
+#include <string.h>
 #include "global.h"
 #include "trainer_pokemon_sprites.h"
 #include "bg.h"
@@ -30,7 +31,6 @@
 #include "scanline_effect.h"
 #include "sound.h"
 #include "sprite.h"
-#include "string.h"
 #include "strings.h"
 #include "string_util.h"
 #include "task.h"