Fix build with Qt 5.9

Summary:
moc 5.9 errors out when building:
Error: Plugin Metadata file "" could not be opened: file to open is a directory

Same issue and fix as https://phabricator.kde.org/D5392 for khtml.

CCBUG: 377490

Test Plan: builds fine now with Qt 5.9.0rc and also earlier versions.

Reviewers: vriezen, pino

Reviewed By: pino

Subscribers: pino

Differential Revision: https://phabricator.kde.org/D5985