multimedia/avidemux: fix build on armv7 with recent binutils

Recent GNU as doesn't like numerical labels that begin with leading
zeros. Fix the one occurrence of such a label in the bundled FFmpeg to
fix the build on armv7.

MFH: 2024Q4
Approved by: portmgr (build fix blanket)

(cherry picked from commit c80d811ea8a01117147be9485becbee5d6945973)