Mark two implicit ctors as explicit

Summary:
[Folly] Mark two implicit ctors as `explicit`.

They did not need to be implicit.

Reviewed By: andrewjcg

Differential Revision: D5524798

fbshipit-source-id: a50bc6bfc9da2642891faf0aa38a19f46e75b9da

folly/io/IOBuf.cpp

@@ -84,9 +84,7 @@ void takeOwnershipError(bool freeOnError, void* buf,
 namespace folly {
 
 struct IOBuf::HeapPrefix {
-  HeapPrefix(uint16_t flg)
-    : magic(kHeapMagic),
-      flags(flg) {}
+  explicit HeapPrefix(uint16_t flg) : magic(kHeapMagic), flags(flg) {}
   ~HeapPrefix() {
     // Reset magic to 0 on destruction.  This is solely for debugging purposes
     // to help catch bugs where someone tries to use HeapStorage after it has

folly/test/DeterministicScheduleTest.cpp

@@ -316,7 +316,7 @@ struct AnnotatedAtomicCounter : public Base<T> {
   }
 
   /* Constructor -- calls original constructor */
-  AnnotatedAtomicCounter(int val) : Base<T>(val) {}
+  explicit AnnotatedAtomicCounter(int val) : Base<T>(val) {}
 
   /* Overloads of original member functions (as needed) */