Log Message:
multimedia/ffmpeg: Fix UINT64_C error on DragonFly

ffmpeg itself built fine on DragonFly, but other code trying to use this
library (e.g. audio/akode-plugins-ffmpeg) would not build, stopping with
an error like:
  common.h:154 error: `UINT64_C` was not declared in this scope

The provided patch fixes this breakage.  For conservative reasons, the
code only applies to DragonFly, but if other platforms are seeing the
same error, then the patch's macro should be expanded to include those
other platforms as well.