Move assert to static_assert

ClangTidy pointed this out.

Change-Id: I59f34bb58a0336fe30fe5f4ce6e118a1b47bbb87
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/823734
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>