net/owamp: Unconditionally define a private timespecadd

net/owamp assumes that if timespecadd is defined, it's identical
to the two-argument version found in the FreeBSD kernel. However,
the three argument version found in NetBSD and OpenBSD is actually
more common. An upcoming commit to head will replace FreeBSD's
version with the NetBSD version. So net/owamp shouldn't assume
that if FreeBSD defines timespecadd, it's defining the 2-argument
version.

PR: 230059
Submitted by: asomers