webkitgtk24: fix gstreamer dependency

Fixes:
http://autobuild.buildroot.net/results/a28/a280f23cc29f77ab440edbda05b4e58d6f4856e7/



The .mk file was checking for gst1-plugins-base but then adding
gst1-plugins-good to dependencies.

According to Source/autotools/FindDependencies.m4, the video and web audio
options need the .pc files provided by gst1-plugins-base, so use that.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>