glsl: don't expose imageAtomicIncWrap for signed image
The spec says that it's only allowed for unsigned ones. Same from imageAtomicDecWrap. Fixes: fc0a2e5d ("glsl: add EXT_shader_image_load_store new image functions") Reviewed-by:Marek Olšák <marek.olsak@amd.com> Part-of: <mesa/mesa!5688> (cherry picked from commit 233af4a4)