Revert "tool: we don't need -lrt."

This reverts commit e60e2a483b3153f2de462371713cbf16e2764541. Turns out we do
still use clock_gettime in speed.cc.

Change-Id: Idab42ef2863345c3d1409b5d33c3e36b41739e1f
Reviewed-on: https://boringssl-review.googlesource.com/4894
Reviewed-by: Adam Langley <agl@google.com>