Use <resolv_iface.h> instead of <resolv.h>

The declarations for _resolv_set_default_iface() and others were
initially added to a public C library header (<resolv.h>). This was a
mistake and they have been moved to a private header
(bionic/libc/private/resolv_iface.h)

This patch updates netd to use the correct private header.

Change-Id: I186390ea3675685b4e0dd5b8a099bd21911f059e