Bug 1564775 [wpt PR 17586] - Restore webidl2.js.headers., a=testonly

Automatic update from web-platform-tests
Restore webidl2.js.headers.

It was added in #6685 and removed accidentally in #16864.

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wpt-commits: 3387cf524436af6e29ad89b1b04ec94b8a76d4f9
wpt-pr: 17586

testing/web-platform/tests/resources/webidl2/lib/webidl2.js.headers

@@ -0,0 +1 @@
+Content-Type: text/javascript; charset=utf-8