Fix dn_comp prototype and add stub

This function is used by ping6 from iputils.

src/network/dn_comp.c

@@ -0,0 +1,9 @@
+#include <resolv.h>
+#include "libc.h"
+
+int __dn_comp(const char *src, unsigned char *dst, int space, unsigned char **dnptrs, unsigned char **lastdnptr)
+{
+	return -1;
+}
+
+weak_alias(__dn_comp, dn_comp);