sdl: forward-declare fscanf_wrapper

Silences a few warnings (if you enable them).

Change-Id: Id9a9f4c688742c13ee3eb0777c40b4d4edcda224
diff --git a/apps/plugins/sdl/include/SDL_config_rockbox.h b/apps/plugins/sdl/include/SDL_config_rockbox.h
index 2b7f9a6..2cefc24 100644
--- a/apps/plugins/sdl/include/SDL_config_rockbox.h
+++ b/apps/plugins/sdl/include/SDL_config_rockbox.h
@@ -203,6 +203,7 @@
 int printf_wrapper(const char*, ...);
 int sprintf_wrapper(char*, const char*, ...);
 int vprintf(const char *fmt, va_list ap);
+int fscanf_wrapper(FILE *f, const char *fmt, ...);
 void fatal(char *fmt, ...);
 void rb_exit(int rc);
 void rbsdl_atexit(void (*)(void));