Fix non-IPv6 build post 57785538c6e0d7e8ca0f161ab95bae10fd304047.
57785538c6e0d7e8ca0f161ab95bae10fd304047 change the test for FreeBSD
from FreeBSD_version to FreeBSD. However this test was performed
before sys/param.h was included, therefore FreeBSD_version was never
defined. As the test was never true opt_random_ip_id.h was never included.
Submitted by: bdragon
Reported by: bdragon
MFC after: 1 week
X-MFC with: 57785538c6e0d7e8ca0f161ab95bae10fd304047