Add entry for 5.15 kernel back to manifest

Kernel 5.15 is still used by EdgeOS, and needs to remain in the
manifest.

BUG=b/408948247
TEST=repo sync --manifest-name=$HOME/cos/manifest/external_full.xml
RELEASE_NOTE=None

Change-Id: I51180b5907f7833889ed2d7c83fb71885b828395
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/98567
Reviewed-by: Kirill Spitsyn <ksp@google.com>
Reviewed-by: Michael Kochera <kochera@google.com>
Tested-by: Kevin Berry <kpberry@google.com>
Reviewed-by: Vineet Bharot <vineetbharot@google.com>
Reviewed-by: Scott Haiden <scotthaiden@google.com>
diff --git a/external_full.xml b/external_full.xml
index 10cbe5a..24d48b3 100644
--- a/external_full.xml
+++ b/external_full.xml
@@ -192,6 +192,10 @@
   <project path="src/third_party/toolbox"
            name="third_party/toolbox"
            revision="refs/heads/main" />
+  <!-- Used by EdgeOS. Check with them before removing. -->
+  <project path="src/third_party/kernel/v5.15"
+           name="third_party/kernel"
+           revision="refs/heads/cos-5.15" />
   <project path="src/third_party/kernel/v6.1"
            name="third_party/kernel"
            revision="refs/heads/cos-6.1" />