spec: add dependency on libreport-plugin-mailx

The default post-create event runs reporter-mailx, which is not in the
dependencies.

Closes rhbz#984065.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Merge pull request #722 from abrt/rhbz984065-rhel6

spec: add dependency on libreport-plugin-mailx