inkscape: fix darwin build

Cairo is also a dependency now.

    /tmp/nix-build-inkscape-0.92.4.drv-0/inkscape-0.92.4/src/display/drawing-context.h:20:10: fatal error: 'cairo.h' file not found
    #include <cairo.h>
             ^~~~~~~~~
    1 error generated.

pkgs/applications/graphics/inkscape/default.nix

@@ -1,7 +1,7 @@
 { stdenv, fetchurl, pkgconfig, perlPackages, libXft
 , libpng, zlib, popt, boehmgc, libxml2, libxslt, glib, gtkmm2
 , glibmm, libsigcxx, lcms, boost, gettext, makeWrapper
-, gsl, gtkspell2, python2, poppler, imagemagick, libwpg, librevenge
+, gsl, gtkspell2, cairo, python2, poppler, imagemagick, libwpg, librevenge
 , libvisio, libcdr, libexif, potrace, cmake
 , librsvg, wrapGAppsHook
 }:
@@ -52,7 +52,8 @@ stdenv.mkDerivation rec {
     librsvg # for loading icons
 
     python2Env perlPackages.perl
-  ] ++ stdenv.lib.optional (!stdenv.isDarwin) gtkspell2;
+  ] ++ stdenv.lib.optional (!stdenv.isDarwin) gtkspell2
+    ++ stdenv.lib.optional stdenv.isDarwin cairo;
 
   enableParallelBuilding = true;