travis: Disable warning that causes a false positive in Xcode 8.3+

travis: Disable warning that causes a false positive in Xcode 8.3+

Xcode 8.3, to which there recently was a switch, spits out a warning for
the potentially unaligned access to ip6_plen in ip-packet.c, which we
explicitly read via untoh16() hence the access to that pointer is not
actually unaligned.  It seems the compiler is not able to determine that
there is no unaligned access even though the function is defined in the
header and marked inline.