SDL_waylanddatamanager.c (mime_data_list_add): constify the buffer param

Fixes -Wdiscarded-qualifiers warnings from Wayland_data_source_add_data()

Xcode/SDL/SDL.xcodeproj/project.pbxproj

@@ -181,7 +181,7 @@ mime_data_list_find(struct wl_list* list,
 static int
 mime_data_list_add(struct wl_list* list, 
                    const char* mime_type,
-                   void* buffer, size_t length)
+                   const void* buffer, size_t length)
 {
     int status = 0;
     size_t mime_type_length = 0;