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