chromium: Build using libexif from nixpkgs.

While libexif has been bundled with chromium for some months already, they only
recently added the GYP option to switch to using the system library. So, let's
enable it.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/applications/networking/browsers/chromium/default.nix

@@ -5,6 +5,7 @@
 , libevent, expat, libjpeg
 , libpng, libxml2, libxslt
 , xdg_utils, yasm, zlib
+, libexif
 
 , python, perl, pkgconfig
 , nspr, udev, krb5
@@ -55,6 +56,7 @@ let
     use_system_flac = true;
     use_system_libevent = true;
     use_system_libexpat = true;
+    use_system_libexif = true;
     use_system_libjpeg = true;
     use_system_libpng = true;
     use_system_libxml = true;
@@ -78,6 +80,7 @@ let
     libevent expat libjpeg
     libpng libxml2 libxslt
     xdg_utils yasm zlib
+    libexif
   ];
 
   maybeSeccompPatch = let