minor #36213 add missing gitattributes for phpunit-bridge (Tobion)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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add missing gitattributes for phpunit-bridge

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       |
| License       | MIT
| Doc PR        |

Seems like the phpunit bridge has been forgotten in symfony/symfony#33579

Commits
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d4c052a2fa add missing gitattributes for phpunit-bridge

Author: fabpot

.gitattributes

@@ -1,3 +1,4 @@
 /Tests export-ignore
 /phpunit.xml.dist export-ignore
+/.gitattributes export-ignore
 /.gitignore export-ignore