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