doxygen: Fix doxygen/gbm.doxy TAGFILES

There has never been a doxygen/gbm_setup output folder.

Appears to have been a copy-paste error from original commit
in 245341f406bbdf23fec3d8ea8ad95c147c11ad1c.

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