kbuild: incremental tags update for GNU Global

GNU gtags support '-i' for updating tag files incrementally.
It runs more quickly than generating new tags after kernel source update.

Signed-off-by:  Jianbin Kang <kjbmail@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/tags.sh

@@ -116,7 +116,7 @@ docscope()
 
 dogtags()
 {
-	all_sources | gtags -f -
+	all_sources | gtags -i -f -
 }
 
 exuberant()