multimedia/gstreamer1-libav: Fix build on aarch64

Fix build on aarch64 when FFMPEG option is off.
This is similar to the fix in multimedia/ffmpeg: svn port r507608

PR: 256935
Approved by: portmgr (build fix blanket)

(cherry picked from commit 905dfe641b7045ca5b92dab3f11acda51d380134)