Fix WM_SIZE_HINTS regression

Bug: 12176

If, upon mapping, WM_SIZE_HINTS specifies a non-resizable window, xfwm4
ignores all subsequent updates to WM_SIZE_HINTS set by the application.

This is a regression introduced by commit 78b5c3c4 (bug 10910).

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>