util: include u_endian.h in u_math.h

u_endian.h needs to be included, otherwise PIPE_ARCH_BIG_ENDIAN might not
be defined on big-endian architectures and the endian conversion macros
will be incorrect.

I don't think anything is broken because of this, I just noticed this when
looking at the file.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>