testsuite/gtk/builder.c: Fix running on Visual Studio

Explicitly say in the code that we are exporting the symbols so that when the
test program runs, it can find the required builder symbols.

testsuite/gtk/builder.c

@@ -2448,13 +2448,13 @@ test_transforms (void)
   g_object_unref (builder);
 }
 
-char *
+G_MODULE_EXPORT char *
 builder_get_search (gpointer item)
 {
   return g_strdup (gtk_string_filter_get_search (item));
 }
 
-char *
+G_MODULE_EXPORT char *
 builder_copy_arg (gpointer item, const char *arg)
 {
   return g_strdup (arg);