xfce4-mixer 4.6.1: add gst-plugins-base dependency

xfce4-mixer 4.6.1: add gst-plugins-base dependency

Without that fix we have:
  checking for gstreamer-plugins-base-0.10 >= 0.10.2... not found
  *** The required package gstreamer-plugins-base-0.10 was not found on your system.
  *** Please install gstreamer-plugins-base-0.10 (atleast version 0.10.2) or adjust
  *** the PKG_CONFIG_PATH environment variable if you
  *** installed the package in a nonstandard prefix so that
  *** pkg-config is able to find it.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>