Add OpenMW-CS RC file to app rather than static lib

Static libraries on Windows can't have embedded resources, so this mean the icon for the CS wasn't used. This could have also been resolved by explicitly requesting the library type as OBJECT rather than letting it default to STATIC (as object libraries aren't a thing on-disk and are just an abstraction in CMake so you can use the same object files in different targets), but this seemed less invasive.

I also made it Win32-only as a Windows .rc file is meaningless on Unix, but it shouldn't be MSVC-only as MinGW can consume them.

Based on the report we got through Discord https://discord.com/channels/260439894298460160/528647692088246302/1193011129090330744, and also checking my 0.48 install, this is a regression since 0.48, so doesn't need a changelog entry.