.pc file does not work on mingw

[Javier Sagredo]

The Cflags of this package include a `-I/mingw64/include` which is not translated by `pkgconf` to a Windows path leading to further errors later on (in this case in ghc-pkg).

See https://github.com/haskell/cabal/issues/9479#issuecomment-1826772061

(Maybe I already opened a conversation in this google groups before logging in but I don’t see it in the list. Sorry if that is the case!)