Expose __res_randomid.

I don't think this will help the original submitter, because <resolv.h>
is mostly useless on Android. One day we might have a deprecation and
removal policy for the NDK, but until then we should probably maintain
this header.

Bug: https://github.com/android-ndk/ndk/issues/739
Test: ran tests
Change-Id: I2db859abfdebc3824c997462fa6133d5b355a8dd
diff --git a/libc/include/resolv.h b/libc/include/resolv.h
index d6db4cf..1518475 100644
--- a/libc/include/resolv.h
+++ b/libc/include/resolv.h
@@ -59,6 +59,9 @@
 int res_query(const char* __name, int __class, int __type, u_char* __answer, int __answer_size);
 int res_search(const char* __name, int __class, int __type, u_char* __answer, int __answer_size);
 
+#define res_randomid __res_randomid
+u_int __res_randomid(void) __INTRODUCED_IN_FUTURE;
+
 __END_DECLS
 
 #endif