resolv: make internal symbols static/hidden

Change-Id: I988b83613e6252c0cc961555e81c10f856a38b37
diff --git a/libc/netbsd/resolv/res_state.c b/libc/netbsd/resolv/res_state.c
index 3a2301d..3209b6f 100644
--- a/libc/netbsd/resolv/res_state.c
+++ b/libc/netbsd/resolv/res_state.c
@@ -134,6 +134,7 @@
     return rt;
 }
 
+__LIBC_HIDDEN__
 struct __res_state _nres;
 
 #if 0