Currently `polipo.h` uses the conditional
`(__GLIBC__ > 2) || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 2)` to decide
whether to enable IPv6 support.

This used to work for OpenWrt CC which uses uClibc disguising itself
as Glibc 2.x but it does not work with Musl libc anymore as this library
does not export any Glibc defines.

Forcibly enable IPv6 support by passing `-DHAVE_IPv6` unconditionally
through the build flags in the OpenWrt/LEDE Makefile.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>