gdkkeys-win32c: change registry key not found g_warning to g_debug

On Windows with certain keyboard layout combinations you can get a
benign terminal warning like this:
Could not open registry key 'SYSTEM\CurrentControlSet\Control\Keyboard
Layouts\D0010413'
When using a lot of plug-ins, like GIMP does, this warning can show up
a lot of times.
The code after this warning, still has another last check, that, when it
fails, will show a warning.

We change the first g_warning to a g_debug statement, to reduce the
amount of warning messages.

Closes #5109

gdk/win32/gdkkeys-win32.c

@@ -249,8 +249,8 @@ _get_keyboard_layout_file (const char *layout_name)
                           KEY_QUERY_VALUE, &hkey);
   if (status != ERROR_SUCCESS)
     {
-      g_warning("Could not open registry key '%s'. Error code: %d",
-                kbdKeyPath, (int)status);
+      g_debug("Could not open registry key '%s'. Error code: %d",
+              kbdKeyPath, (int)status);
       goto fail1;
     }