meta-facebook: 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).

(From meta-facebook rev: 056ec6f35e906dbc6eb2493bd8e26cb28d74d0ba)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I101c12f361c042cc2eb6d0d7373295e62dac028a
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/meta-facebook/conf/layer.conf b/meta-facebook/conf/layer.conf
index 7ff4ac1..aa0e82f 100644
--- a/meta-facebook/conf/layer.conf
+++ b/meta-facebook/conf/layer.conf
@@ -7,4 +7,4 @@
 
 BBFILE_COLLECTIONS += "facebook-layer"
 BBFILE_PATTERN_facebook-layer := "^${LAYERDIR}/"
-LAYERSERIES_COMPAT_facebook-layer := "zeus dunfell"
+LAYERSERIES_COMPAT_facebook-layer := "dunfell gatesgarth"