Solaris 10 compatibility patch

Solaris 10 compatibility patch

Solaris 10's version of netinet/ip.h doesn't include a definition for
IPTOS_MINCOST, which results in the following build error:

---
libtool: link: ( cd ".libs" && rm -f "liboping.la" && ln -s
"../liboping.la" "liboping.la" )
gcc -DHAVE_CONFIG_H -I.   -D_XPG4_2 -D__EXTENSIONS__ -Wall -Werror -g
-O2 -c oping.c
oping.c: In function `usage_qos_exit':
oping.c:311: error: `IPTOS_MINCOST' undeclared (first use in this function)
oping.c:311: error: (Each undeclared identifier is reported only once
oping.c:311: error: for each function it appears in.)
oping.c: In function `set_opt_send_qos':
oping.c:389: error: `IPTOS_MINCOST' undeclared (first use in this function)
gmake[3]: *** [oping.o] Error 1
---

I've attached a patch to define this value, consistent with how the
Asterisk project resolved this issue:
https://issues.asterisk.org/view.php?id=12050

The patch was developed against liboping 1.6.2; it doesn't appear
there have been any changes in git since this release.

Thanks,
--Scott Severtson

Signed-off-by: Florian Forster <octo@verplant.org>