manifest: Checkout master-0.22 for pyelftools

That's what Chromium OS does:
https://chromium.googlesource.com/chromiumos/manifest/+/refs/heads/master/full.xml#67.

BUG=b/151985537
TEST=`build_image --board lakitu test` now works.

Change-Id: Ibfc26d0bb9f94af0cfeda325f357a10da6261897
diff --git a/external_full.xml b/external_full.xml
index 4387582..6a65a36 100644
--- a/external_full.xml
+++ b/external_full.xml
@@ -31,7 +31,8 @@
   <project path="chromite/infra/proto"
            name="mirrors/cros/chromiumos/infra/proto" />
   <project path="chromite/third_party/pyelftools"
-           name="mirrors/cros/chromiumos/third_party/pyelftools" />
+           name="mirrors/cros/chromiumos/third_party/pyelftools"
+           revision="refs/heads/master-0.22" />
   <project path="src/platform/crostestutils"
            name="mirrors/cros/chromiumos/platform/crostestutils" />
   <project path="src/scripts"