Fix redefinition of struct in6_addr

Fix redefinition of struct in6_addr

struct in6_addr is both defined in the C library header <netinet/in.h>
and the Linux kernel header <linux/in6.h>.

lxc_user_nic.c includes both <netinet/in.h> and <linux/if_bridge.h>. The
later one includes <linux/in6.h>.

This breaks build with the musl libc:
  error: redefinition of ‘struct in6_addr’

As lxc_user_nic.c does not use any references from <linux/if_bridge.h> it
is safe to remove this header.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>