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>