windows.mingw_w64: enable stripping

With 0507725 "setup-hooks/strip.sh: run RANLIB on static
archives after stripping" it should now be safe to run strip
on wider range of .a files.

mingw-w64 is a good example where a reference was preserved
to gcc from intermediate stage. Enabling stripping by default
decreases `pkgsCross.mingw32` closure for 20%:

Before:

$ nix path-info -rsSh $(nix-build -A pkgsCross.mingw32.stdenv)
...
/nix/store/qzhkidff0wxhqf2gi97ng6qismzvpnbp-stdenv-linux 43.6K 1.0G

After:

$ nix path-info -rsSh $(nix-build -A pkgsCross.mingw32.stdenv)
...
/nix/store/fj4dv1n3sa3jgcb1j3nwn6njsf943n48-stdenv-linux 43.6K 792.4M

pkgs/os-specific/windows/mingw-w64/default.nix

···
   enableParallelBuilding = true;
 
   buildInputs = [ windows.mingw_w64_headers ];
-  dontStrip = true;
   hardeningDisable = [ "stackprotector" "fortify" ];
 
   meta = {