libweston/color-management: Add fallback for static_assert

Attempt to fix the following with clang-18:

../weston-9999/libweston/color-management.c:890:2: error: call to
undeclared function 'static_assert'; ISO C99 and later do not support
implicit function declarations [-Wimplicit-function-declaration] 890 |
static_assert(UINT32_MAX <= SIZE_MAX, |         ^

Fixes: #948

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>