tool: we don't need -lrt.

I think, long ago, I tried to use the monotonic clock in speed.cc, which
needs -lrt. However, the current code doesn't use that and thus doesn't
need -lrt.

Change-Id: Ibcbf90f91ae6b852c0975dff006346125243df54
Reviewed-on: https://boringssl-review.googlesource.com/4622
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>