[icons] Label them as vendored for GitHub (#22397)

Author: oliviertassinari

.gitattributes

@@ -3,3 +3,6 @@
 # Undo the GitHub's default
 # https://github.com/github/linguist/blob/96ad1185828f44bb9b774328a584551ee57ed264/lib/linguist/vendor.yml#L177
 packages/**/*.d.ts -linguist-vendored
+# These icons are imported from Google.
+# Kept so that we can review the impact when changing the generation script.
+packages/material-ui-icons/src/*.js linguist-vendored