chromium: Move libexif to propagatedBuildInputs.

This is needed especially for Chromium version 36 and higher, because
libexif is dlopen()ed over there.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
chromium: Don't add sources as runtime dependency.

This is because of a single file that symlinks to the source output
path:

libexec/chromium/resources/extension/demo/library.js

Target within source output path:

chrome/browser/resources/extension_resource/demo/library.js

So we just need to ensure that the cp command follows symlinks during
installPhase and we should no longer have this unnecessary dependency.

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