Auto merge of #882 - Manishearth:gstreamer-doc, r=jdm
Auto merge of #882 - Manishearth:gstreamer-doc, r=jdm

Fix doc build with gstreamer

Doc builder needs the gstreamer deps too.

fixes servo/servo#21416

Untested since I have to wait for buildbot to stop testing things to reload it.

r? @jdm

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servo/saltfs,servo/saltfs