VITA: Fix "conflicting types for LoadAchievements" warning

source/menu.c: At top level:
source/menu.c:1300:6: warning: conflicting types for 'Load_Achivements'
 1300 | void Load_Achivements (void)
      |      ^~~~~~~~~~~~~~~~
source/menu.c:604:2: note: previous implicit declaration of 'Load_Achivements' was here
  604 |  Load_Achivements();
      |  ^~~~~~~~~~~~~~~~

source/menu.c

@@ -601,6 +601,7 @@ void M_Menu_Main_f (void)
 	m_entersound = true;
 	
 #ifdef VITA
+	void Load_Achivements(void);
 	Load_Achivements();
 #endif
 }