gl_table.py: add extern C guard for the generated glapitable.h

The header can be included from C++, hence contents should have
appropriate notation.

Cc: mesa-stable@lists.freedesktop.org
Cc: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
(cherry picked from commit c7616ac0)