kas: update meta-clang to use the gatesgarth branch

When the CI was written there wasn't a gatesgarth branch, but there is now
and it is needed as master meta-clang doesn't work with gatesgarth oe-core.

Change-Id: Ief06b6c5be11dd7fc77520f5601b662bdca52891
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

kas/clang.yml

@@ -4,8 +4,7 @@ header:
 repos:
   meta-clang:
     url: https://github.com/kraj/meta-clang
-    # This doesn't (yet) have a gatesgarth branch
-    refspec: master
+    refspec: gatesgarth
 
 local_conf_header:
   clang: |