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: NMichael Tokarev <mjt@tls.msk.ru>

configure

@@ -1723,6 +1723,7 @@ fi
 
 cat > $TMPC <<EOF
 #include <sys/socket.h>
+#include <linux/ip.h>
 int main(void) { return sizeof(struct mmsghdr); }
 EOF
 if compile_prog "" "" ; then