Fix wrong path of afxres.h by using $(VCToolsInstallDir)

Languages/TortoisePot.vcxproj uses $(VCInstallDir)atlmfc\include\afxres.h, but it can't locate the afxres.h of VS2017 Community any more. Because:

$(VCInstallDir)atlmfc\include\afxres.h
=
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\atlmfc\include\afxres.h

but correct place is

C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\atlmfc\include\afxres.h

$(VC_ATLMFC_IncludePath) was my way to fix it, but after the testing:

$(VC_ATLMFC_IncludePath)\afxres.h
=
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\atlmfc\include;\afxres.h

Extra semicolon is appended to $(VC_ATLMFC_IncludePath), and make the path wrong way.

The next testing is using $(VCToolsInstallDir), and it works. :-)