package/gstreamer1/gst1-python: needs gst1-plugins-base

gst1-plugins-base is a mandatory dependency since at least version
1.9.90 and
https://github.com/GStreamer/gst-python/commit/16f971226df1980b58ebde330123debaaf3b53d0

Fixes:
 - http://autobuild.buildroot.org/results/48b22c66c3a610d70931b9adfd6e5082bb3ff3d1



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>