[AUTOMOC] Call moc.exe less times when using UNITY_BUILD

Profiling a Windows/MSVC build showed the build spend 15 seconds just running hundreds of moc.exe instances , even though I'm using a UNITY_BUILD.

Running a single moc invocation on the unity of all headers would improve build times on Windows.

moc is usually fast by itself but Windows is famous for taking much more time to create a new process (which is also probably why UNITY_BUILD gives a bigger boost on Windows).