gitattributes: Require LF endings for .proto files.

This was likely missed earlier, but was noticed during https://crrev.com/c/2401500.

Bug: 832129
Change-Id: Id808812aefc73cf3218db3ee915197b4da7937c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2402101
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805500}

.gitattributes

@@ -27,6 +27,7 @@
 *.mm text eol=lf
 *.mojom text eol=lf
 *.pdf -diff
+*.proto text eol=lf
 *.sh text eol=lf
 *.sql   text eol=lf
 *.txt   text eol=lf