fix linking ffmpeg library on android

When ffmpeg is built with --target-os=android there is no need for manual specification of library version. Actually, that only creates a problem when different version of libraries is compiled than specified here.

wiki page is updated to reflect this change:
https://github.com/wang-bin/QtAV/wiki/Make-app-for-Android
Merge pull request #276 from pkoretic/master

fix linking ffmpeg library on android