Help automoc to find metadata JSON files referenced in the code

Summary:
automoc by itself can only detect metadata files referenced by direct
usage of the Q_PLUGIN_METADATA macro. It does not do any C++ preprocessor
evaluation. Instead it needs to be helped with regexp-based filter rules for
detecting any names of files used as additional moc input.
See docs for AUTOMOC_DEPEND_FILTERS for further details.

Test Plan:
Changing a JSON file and running make results in the related *.moc file being
regenerated and the related object file being recompiled.
qtplugininfo shows that the created plugin binary has up-to-date JSON content.

Reviewers: #kdevelop, apol, kfunk

Reviewed By: #kdevelop, apol

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D10693