meson: Only ensure that moc is available on Linux
On other OSes, it's not possible to have qmake or the qt5 pkg-config
files and not have moc, and moc will not be in PATH, so this only
causes problems.
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On other OSes, it's not possible to have qmake or the qt5 pkg-config
files and not have moc, and moc will not be in PATH, so this only
causes problems.