Log Message:
multimedia/ffmpeg: rework UINT64_C fix for DragonFly and NetBSD

Due to multiple inclusions of stdint.h, DragonFly was failing to build
one of the akode plugins with a typedef redefinition error.  It had
previously built, so the UINT64_C fix for the akode mpc plugin broke
the ffmpeg plugin.

It turns out that the first patch effectively just imported
machine/int_const.h, so that header can be directly imported instead
rather than having to include the main contents of <stdint.h> multiple
times.  This refinement also works on NetBSD, but OpenBSD and FreeBSD
unfortunately embed UINT64_C directly within <stdint.h>.  Therefore
original patch is left in place to cover those systems.