doxygen: Plumb through gallium/ to automated documentation

Add Gallium and the Gallium-based drivers to doxygen's automated
code documentation infrastructure.

Can be individually created with:

  cd $MESA_TOP_LEVEL/
  make -C doxygen/ gallium.tag

Benefits from the existing doxygen Makefile runners to clean up
afterwards with 'make clean'.

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

doxygen/Makefile

@@ -25,7 +25,8 @@ full: $(FULL:.doxy=.tag)
 
 SUBSET = \
 	main.doxy \
-	math.doxy
+	math.doxy \
+	gallium.doxy
 
 subset: $(SUBSET:.doxy=.tag)
 	$(foreach FILE,$(SUBSET),doxygen $(FILE);)