syntax-check: fix cast_of_argument_to_free

In this case, make syntax-check is wrong, and we actually do
need the cast.
A cast is needed when types are   uint64_t <--> pointer

Using a local "ptr" variable makes both gcc and syntax-check happy.

server/tests/test_display_base.c

@@ -42,7 +42,8 @@ static void test_spice_destroy_update(SimpleSpiceUpdate *update)
         return;
     }
     if (update->drawable.clip.type != SPICE_CLIP_TYPE_NONE) {
-        free((uint8_t*)update->drawable.clip.data);
+        uint8_t *ptr = (uint8_t*)update->drawable.clip.data;
+        free(ptr);
     }
     free(update->bitmap);
     free(update);