Tests: fix calling gdk_init()

gdk_init() takes pointers to these arguments so it can modify them to strip all
arguments already consumed.

Tests/FindGTK2/gdk/main.c

@@ -2,6 +2,6 @@
 
 int main(int argc, char* argv[])
 {
-  gdk_init(argc, argv);
+  gdk_init(&argc, &argv);
   return 0;
 }