l2tpv3 (configure): it is linux-specific

l2tpv3 (configure): it is linux-specific

Some non-linux systems, for example a system with
FreeBSD kernel and glibc, may declare struct mmsghdr
(in glibc) but may not have linux-specific header
file linux/ip.h.  The actual implementation in qemu
includes this linux-specific header file unconditionally,
so compilation fails if it is not present.  Include
this header in the configure test too.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(cherry picked from commit bff6cb72961f1bd2c766efe85ff5850fd8d7e77d)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>