musl-compat: avoid kernel header conflicts

The conflict between Musls net/if.h and linux/if.h is an old well known
one and taken care of by a series of linux-headers patches in OpenWrt.
Since Linux 4.8-rc5 Firewall3 also indirectly pulls in linux/in.h and
linux/in6.h leading to new conflicts.

As Firewall3 is fine with just the libc headers prevent inclusion of the
corresponding kernel headers.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>