Add missing clang-format commit to .git-blame-ignore-revs

After 19d98d6a reformatted Okular's codebase with `clang-format`, that
commit was marked to be ignored by `git blame` by adding it to
`.git-blame-ignore-revs` in 4fb45522.

Later, !296 was merged but still needed reformatting, which was done in
b09e23f6. However, the latter commit was still missing from
`.git-blame-ignore-revs`, so let's add it.

Test Plan:
All commits mentioning `clang-format` which reformatted the codebase are
now included in `.git-blame-ignore-revs`.

.git-blame-ignore-revs

@@ -1,2 +1,4 @@
 # _clang_format added
 19d98d6a74079a42bcc5cb0e899d239a4bdb6e20
+b09e23f6ab7bf806448d87df1ceb1ea769d5a300
+