Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I84effbdfa1b9b39328a909b7f70fe17e7ee316c8
diff --git a/tests/time_test.cpp b/tests/time_test.cpp
index b1f6364..10399c5 100644
--- a/tests/time_test.cpp
+++ b/tests/time_test.cpp
@@ -273,7 +273,7 @@
   }
 
  public:
-  Counter(void (*fn)(sigval_t)) : value(0), timer_valid(false) {
+  explicit Counter(void (*fn)(sigval_t)) : value(0), timer_valid(false) {
     memset(&se, 0, sizeof(se));
     se.sigev_notify = SIGEV_THREAD;
     se.sigev_notify_function = fn;