meta-hxt: update Yocto compat for gatesgarth

The next release of Yocto is soon and most of the upstream
layers have switched support strings for it.  Support layer
compat for gatesgarth (current) and dunsfell (previous).

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I3b5ef37d96be06be175e35ad689546bf8d584601
diff --git a/meta-hxt/conf/layer.conf b/meta-hxt/conf/layer.conf
index bc33c2b..5de4403 100644
--- a/meta-hxt/conf/layer.conf
+++ b/meta-hxt/conf/layer.conf
@@ -3,3 +3,5 @@
 
 BBFILE_COLLECTIONS += "hxt"
 BBFILE_PATTERN_hxt = ""
+
+LAYERSERIES_COMPAT_hxt = "dunfell gatesgarth"