VS: Fix compilation of single source with PCH in Unity Build
Sources that are part of a unity build are normally not compiled individually. However, the VS IDE allows a single source to be compiled. This can also be achieved on the command line:
msbuild my.vcxproj ... -t:ClCompile -p:SelectedFiles=<src>
where <src>
is the path in the vcxproj ClCompile
entry.
In a target with precompiled headers, the source needs PCH settings to support individual compilation even if the normal unity build does not.