Bug#842840: libavfilter-extra6: should provide libavfilter6

[Jonas Smedegaard]

Package: libavfilter-extra6
Severity: important

libavfilter-extra6 is a superset of libavfilter6, and should therefore
declare that it "Provides: libavfilter6" to not effectively conflict
with all consumers of libavfilter6.

 - Jonas