CMake: Link `time_zone` library to `Threads::Threads`

`time_zone_impl.cc` uses `std::mutex`.
diff --git a/absl/time/CMakeLists.txt b/absl/time/CMakeLists.txt
index b312425..1c830c7 100644
--- a/absl/time/CMakeLists.txt
+++ b/absl/time/CMakeLists.txt
@@ -84,6 +84,7 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    Threads::Threads
     $<$<PLATFORM_ID:Darwin>:${CoreFoundation}>
 )