inetutils_1.8.bb: Fix build when DISTRO_FEATURES lacks ipv6.

inetutils_1.8.bb: Fix build when DISTRO_FEATURES lacks ipv6.

In file included from ./ping.h:19:0,
                 from ping_address.c:47:
./ping_common.h:23:27: fatal error: netinet/icmp6.h: No such file or directory
compilation terminated.
In file included from ./ping.h:19:0,
                 from ping_router.c:46:
./ping_common.h:23:27: fatal error: netinet/icmp6.h: No such file or directory
compilation terminated.
make[2]: *** [ping_address.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: *** [ping_router.o] Error 1
In file included from ping_common.c:32:0:
ping_common.h:23:27: fatal error: netinet/icmp6.h: No such file or directory
compilation terminated.
make[2]: *** [ping_common.o] Error 1
In file included from ping.h:19:0,
                 from libping.c:38:
ping_common.h:23:27: fatal error: netinet/icmp6.h: No such file or directory
compilation terminated.
make[2]: *** [libping.o] Error 1
In file included from ./ping.h:19:0,
                 from ping_timestamp.c:46:
./ping_common.h:23:27: fatal error: netinet/icmp6.h: No such file or directory
compilation terminated.
make[2]: *** [ping_timestamp.o] Error 1
In file included from ./ping.h:19:0,
                 from ping_echo.c:47:
./ping_common.h:23:27: fatal error: netinet/icmp6.h: No such file or directory
compilation terminated.
make[2]: *** [ping_echo.o] Error 1
In file included from ./ping.h:19:0,
                 from ping.c:49:
./ping_common.h:23:27: fatal error: netinet/icmp6.h: No such file or directory
compilation terminated.
make[2]: *** [ping.o] Error 1

Signed-off-by: Graham Gower <graham.gower@gmail.com>
Acked-by: Khem Raj <raj.khem@gmail.com>