Unbreak gv (fixes #34841) (#40658)

It appears that without pkgconfig, the build process misdetects some features, resulting in gv to segfault immediately on startup.

The fix was adopted from https://github.com/Homebrew/legacy-homebrew/issues/18555#issuecomment-15074618.

pkgs/applications/misc/gv/default.nix

···
     Xaw3d
     ghostscriptX
     perl
-  ] ++ stdenv.lib.optionals stdenv.isDarwin [
     pkgconfig
+  ] ++ stdenv.lib.optionals stdenv.isDarwin [
     libiconv
   ];