headers: Add exp10 declarations to gnu/math.h.

These are GNU extensions so they need to be behind _GNU_SOURCE.

Fixes #16371.

sincos, sincof, sincosl declarations in gnu/math.h

Signed-off-by: Zoltán Mizsei <zmizsei@extrowerk.com>

Change-Id: I44b39e8d76dd970e18e285b5d8b9bad715608154
Reviewed-on: https://review.haiku-os.org/715
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>