[build/frr] src/sonic-frr/frr is changed after the build

[wdoekes]

Description

After the build src/sonic-frr/frr looks changed. This is cosmetic only, because a rebuild resets the submodule. But it's a small annoyance that we can fix.

Steps to reproduce the issue:

1. make all

Describe the results you received:

1. A built image.
2. And some untracked files.
3. But also a supposed change:

$ git diff
diff --git a/src/sonic-frr/frr b/src/sonic-frr/frr
index de0e358b8..61995a7d8 160000
--- a/src/sonic-frr/frr
+++ b/src/sonic-frr/frr
@@ -1 +1 @@
-Subproject commit de0e358b877ac9b595e7fb387a302c960a4c02d1
+Subproject commit 61995a7d83f385114bd0b929f7401ae0e930dd15

Describe the results you expected:

1. A built image.
2. And some untracked files.
3. But no changed files.

$ git diff
(void)

Fix

See this PR: #20868.