Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/gc/accounting/space_bitmap.h b/runtime/gc/accounting/space_bitmap.h
index 32ab440..5a1bfe3 100644
--- a/runtime/gc/accounting/space_bitmap.h
+++ b/runtime/gc/accounting/space_bitmap.h
@@ -244,7 +244,7 @@
     }
   }
 
-  SpaceSetMap(const std::string& name) : name_(name) {}
+  explicit SpaceSetMap(const std::string& name) : name_(name) {}
   ~SpaceSetMap() {}
 
   Objects& GetObjects() {