Fix an issue where the build failed with vv disabled but gstreamer available.

This is kind of ugly, but mediamanager.c is a spaghetti mess of preproc and
this appeared to be the best/quickest way to fix the issue as
PurpleMediaManager isn't fully defined unless vv is enabled.

Testing Done:
Just a compile test with `--disable-vv` as well as no arguments to configure.

Reviewed at https://reviews.imfreedom.org/r/25/