XCB: Fix pixmap leak

In qt_xcb_XPixmapFromBitmap(), a new pixmap is generated for the caller.
This pixmap has to be freed after use. However,
createNonStandardCursor() didn't do this and instead leaked the pixmap.

Change-Id: I6fee180ec6508db9e82a5bb028957e7d9f7a4632
Signed-off-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>