Somewhere, the declaration of _resolv_delete_cache_for_net went missing.

I couldn't work out where this went missing from "git log", but it became
important with the switch to hidden visibility.

Change-Id: I921f81e36642a2c65c3ec85dc9b5b87c5d977a5b
diff --git a/libc/dns/include/resolv_netid.h b/libc/dns/include/resolv_netid.h
index 63ecdd8..bada18a 100644
--- a/libc/dns/include/resolv_netid.h
+++ b/libc/dns/include/resolv_netid.h
@@ -64,6 +64,9 @@
 /* flush the cache associated with a certain network */
 extern void _resolv_flush_cache_for_net(unsigned netid) __used_in_netd;
 
+/* delete the cache associated with a certain network */
+extern void _resolv_delete_cache_for_net(unsigned netid) __used_in_netd;
+
 /* Internal use only. */
 struct hostent *android_gethostbyaddrfornet_proxy(const void *, socklen_t, int , unsigned);
 int android_getnameinfofornet(const struct sockaddr *, socklen_t, char *, size_t, char *, size_t,