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>
diff --git a/doxygen/gbm.doxy b/doxygen/gbm.doxy
index d0f7f95..8cfe98a 100644
--- a/doxygen/gbm.doxy
+++ b/doxygen/gbm.doxy
@@ -42,7 +42,7 @@
 TAGFILES               = main.tag=../core \
                          math.tag=../math \
                          tnl_dd.tag=../tnl_dd \
-                         swrast_setup.tag=../gbm_setup \
+                         swrast_setup.tag=../swrast_setup \
                          tnl.tag=../tnl \
                          vbo.tag=../vbo
 GENERATE_TAGFILE       = gbm.tag