resolv: make internal symbols static/hidden

Change-Id: I988b83613e6252c0cc961555e81c10f856a38b37
diff --git a/libc/netbsd/resolv/res_cache.c b/libc/netbsd/resolv/res_cache.c
index 2621a7b..84194c2 100644
--- a/libc/netbsd/resolv/res_cache.c
+++ b/libc/netbsd/resolv/res_cache.c
@@ -1143,7 +1143,7 @@
          "*************************");
 }
 
-struct resolv_cache*
+static struct resolv_cache*
 _resolv_cache_create( void )
 {
     struct resolv_cache*  cache;