Add entry for 5.15 kernel in mainifest.

BUG=b/208840137
TEST=repo sync --manifest-file=$HOME/cos/manifest/external_full.xml
RELEASE_NOTE=None

Change-Id: Ibc67ac362ff07249a225efdc6d161498ab8cc9ee
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/27501
Reviewed-by: Oleksandr Tymoshenko <ovt@google.com>
Tested-by: Meena Shanmugam <meenashanmugam@google.com>
diff --git a/external_full.xml b/external_full.xml
index cf7588c..4c2b9ff 100644
--- a/external_full.xml
+++ b/external_full.xml
@@ -105,6 +105,9 @@
   <project path="src/third_party/kernel/v5.10"
            name="third_party/kernel"
            revision="refs/heads/cos-5.10" />
+  <project path="src/third_party/kernel/v5.15"
+           name="third_party/kernel"
+           revision="refs/heads/cos-5.15" />
 
   <repo-hooks enabled-list="pre-upload"
            in-project="cos/repohooks" />