cmake: fix MSVC PCH support

I'm not sure if the previous implementation ever worked.

Source/PCH/nopch/pch.h

@@ -0,0 +1,4 @@
+// dummy include to help with disabling pch for a single file
+// cmake doesn't provide a clean way to disable MSVC's force include option
+
+// So we can just point it at an empty file instead.