fetchpatch: Allow to pass a postFetch attribute.

Comes in handy if we want to make additional modificiations to the
output file. While I wasn't sure whether to invoke the passed postFetch
directly before the patch or afterwards, I thought it would be better
afterwards because "postFetch of fetchpatch" at least to my intuition
would sound that after whatever "fetchpatch" does - it comes afterwards.

Signed-off-by: aszlig <[e-mail]>

pkgs/build-support/fetchpatch/default.nix

···
         --strip=${toString stripLen} \
         --clean "$out" > "$tmpfile"
     mv "$tmpfile" "$out"
+    ${args.postFetch or ""}
   '';
 } // builtins.removeAttrs args ["stripLen"])