Add autoninja* to .gitattributes.

Bug: b/77176746
Change-Id: I8f027daa6f867d61f0a9124a6d22c512bf91f219
Reviewed-on: https://chromium-review.googlesource.com/c/1325267Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>

.gitattributes

@@ -13,6 +13,7 @@
 /gsutil.vpython recipes
 
 # Extensionless tools we want and support scripts.
+/autoninja*                   recipes
 /clang*                       recipes
 /depot-tools-auth             recipes
 /download_from_google_storage recipes