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>
diff --git a/tool/CMakeLists.txt b/tool/CMakeLists.txt
index 4bb6ca2..74c1ac8 100644
--- a/tool/CMakeLists.txt
+++ b/tool/CMakeLists.txt
@@ -15,4 +15,8 @@
   transport_common.cc
 )
 
-target_link_libraries(bssl ssl crypto)
+if (APPLE OR WIN32 OR ANDROID)
+  target_link_libraries(bssl ssl crypto)
+else()
+  target_link_libraries(bssl ssl crypto -lrt)
+endif()