Fix folly/synchronization/Tearable.h under gcc49

Summary: [Folly] Fix `folly/synchronization/Tearable.h` under `gcc49`, which version of `libstdc++` lacks `std::is_trivially_copyable`.

Reviewed By: simpkins

Differential Revision: D6805662

fbshipit-source-id: 5f368cda58b860c7a7ee05addb325a655ab09830

folly/synchronization/Tearable.h

@@ -19,6 +19,8 @@
 #include <cstring>
 #include <type_traits>
 
+#include <folly/Traits.h>
+
 namespace folly {
 
 /**
@@ -48,7 +50,7 @@ class Tearable {
   // We memcpy the object representation, and the destructor would not know how
   // to deal with an object state it doesn't understand.
   static_assert(
-      std::is_trivially_copyable<T>::value,
+      IsTriviallyCopyable<T>::value,
       "Tearable types must be trivially copyable.");
 
   Tearable() {