lp: make sure 0xff is unsigned before shifting it past signed int range

src/gallium/auxiliary/gallivm/lp_bld_format_s3tc.c:2446:82: runtime error: left shift of 255 by 24 places cannot be represented in type 'int'

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26368>
1 file changed