doc: Mention that DragonFly BSD requires __BSD_VISIBLE set
Setting this should fix the FATE build failure on DragonFly BSD,
since inet_aton only is visible if __BSD_VISIBLE is set.
Alternatively, a line defining __BSD_VISIBLE=1 could be
added at the top of os_support.c. For FreeBSD, similar lines
are required in libavdevice/bktr.c and libavdevice/oss_audio.c, too.
Signed-off-by: Martin Storsjö <email@example.com>