os: add missing include for bswap_* on BSDs

os: add missing include for bswap_* on BSDs

Commit 0dcebdf4a70ef0d8144b8fcba763ae87e7fc74b5 ("configure: add endian
check") removed the sys/endian.h include from the os/*.h BSD headers but
this include is needed to access the bswap_* macros. The lack of this
include wouldn't show up unless you were compiling for a big-endian BSD
platform.

This commit adds the include back for {Free,Net,Open}BSD and adds it for
the first time to the DragonFlyBSD header.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>