contrib: ffmpeg: change rules.mak optflags to -O0

Given one try to debug ffmpeg contrib in lldb, when --disable-optim option is used on bootstrap, then -Og flag was added to compiler flags.

With -Og many stack frames were broken (unavailable variables or unexpected program pointer jumps when stepping through code)

Using -O0 instead seems to fix the issue to provide proper stack frame access in lldb.