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