Carlos Garnacho authored Nov 27, 2018 and Jonas Ådahl committed Nov 27, 2018

If meson tries to get ahead and generate object files for tests
at the same time than building libmutter, those may randomly fail
if meson did not create the libmutter generated headers yet.

Add those to the declared dependency, so the files are ensured to
be created before anything gets to use it.

Closes: #404