gui/image/qimage_conversions.cpp:2257:9: error: expected ‘}’ before numeric constant

On big-endian systems, compiling gui/image/qimage_conversions.cpp fails, apparently because the big-endian code path has not been updated along with the little-endian code path. The attached patch addresses that issue.

Previously reported at https://bugs.gentoo.org/show_bug.cgi?id=538570