matplotlib: Use ghostscript_headless

matplotlib does not seem to use ghostscript in its graphical /
interactive capacity, based on quick scan through the docs and
github code search.

If this needs to be reverted, please state the graphical use case
in a comment, so we don't make this mistake again.

pkgs/top-level/python-packages.nix

@@ -5594,6 +5594,7 @@ self: super: with self; {
   matplotlib = callPackage ../development/python-modules/matplotlib {
     stdenv = if stdenv.isDarwin then pkgs.clangStdenv else pkgs.stdenv;
     inherit (pkgs.darwin.apple_sdk.frameworks) Cocoa;
+    ghostscript = pkgs.ghostscript_headless;
   };
 
   matplotlib-inline = callPackage ../development/python-modules/matplotlib-inline { };