BUILD: tcp: use IPPROTO_IPV6 instead of SOL_IPV6 on FreeBSD/MacOS

BUILD: tcp: use IPPROTO_IPV6 instead of SOL_IPV6 on FreeBSD/MacOS

Lukas reported in issue #1203 that the previous fix for silent-drop in
commit ab79ee8b1 ("BUG/MINOR: tcp: fix silent-drop workaround for IPv6")
breaks the build on FreeBSD/MacOS due to SOL_IPV6 not being defined. On
these platforms, IPPROTO_IPV6 must be used instead, so this should fix
it.

This needs to be backported to whatever version the fix above is backported
to.

(cherry picked from commit da2319578555e56eafab0141505ce943f954d859)
Signed-off-by: Willy Tarreau <w@1wt.eu>