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>