iptunnel: drop netinet/ip.h include

The only reason we include this header is to get the IP_DF define.
However, we already have a local #define fallback in case that isn't
already defined for us.

Including this header while also including linux/if_tunnel.h causes
problems with newer Linux headers (v4.8+):
cc -O2 -g -Wall -fno-strict-aliasing  -D_GNU_SOURCE \
	-D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE \
	-I. -I/usr/local/src/net-tools/include -Ilib -c iptunnel.c
In file included from /usr/include/linux/if_tunnel.h:6:0,
                 from iptunnel.c:34:
/usr/include/linux/ip.h:85:8: error: redefinition of 'struct iphdr'
 struct iphdr {
        ^
In file included from iptunnel.c:29:0:
/usr/include/netinet/ip.h:44:8: note: originally defined here
 struct iphdr
        ^

Changing netinet/ip.h to linux/ip.h won't help because it doesn't
provide IP_DF or anything else we care about.  Simply drop the header
to avoid the build failure.
Reported-by: Randy MacLeod <randy.macleod@windriver.com>

iptunnel.c

@@ -26,7 +26,6 @@
 #include <sys/socket.h>
 #include <sys/ioctl.h>
 #include <netinet/in.h>
-#include <netinet/ip.h>
 #include <arpa/inet.h>
 #include <net/if.h>
 #include <net/if_arp.h>