include: drop c99_math.h

include: drop c99_math.h

Since we now depend on C11, we know that we have support for the C99
math functionality. So let's drop the c99_math.h compatibility wrapper,
and just include <math.h> directly.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16812>