Install pkg-config .pc file also on Windows/MSVC

[traversaro]

On Windows, both vcpkg and conda package managers explicitly support the use of pkg-config .pc files. Consuming this files is done either via CMake's FindPkgConfig module, or directly using pkg-config tool with the --msvc-syntax command line option (see https://gitlab.freedesktop.org/pkg-config/pkg-config/-/blob/master/README.win32).

For this reason, I do not think there is any reason for not installing .pc files if the library is configured on Windows.