doxygen: Fix doxygen/glapi.doxy

doxygen: Fix doxygen/glapi.doxy

The src/mesa/glapi folder was relocated in the below commit.
Amend the doxygen/glapi.doxy INPUT setting accordingly.

Whilst here, in addition this change also avoids a bug in the
consolidated Doxygen output caused by doxygen/glapi.doxy inadvertently
overwriting doxygen/swrast.tag via its GENERATE_TAGFILE setting.

This bug depended upon the specific order each *.tag was built.

   commit 296adbd545b8efd38c9ed508166b2de2764a444b
   Author: Chia-I Wu <olv@lunarg.com>
   Date:   Mon Apr 26 12:56:44 2010 +0800

       glapi: Move to src/mapi/.

       Move glapi to src/mapi/{glapi,es1api,es2api}.

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>