plugin: add Apache 2 license to list of known licenses

the licence in gstreamer/subprojects/gstreamer/gst/gstplugin.c currently is defined to be one of:

LGPL GPL QPL GPL/QPL MPL BSD MIT/X11 0BSD Proprietary

The open source project for the kinesis plugin is using an Apache 2.0 license.

Because "Apache 2.0" is not one of the supported licenses it automatically falls back to Proprietary. Would it be possible to add "Apache 2.0" ?

note: the kinesis plugin that caused this merge request