small_vector default constructor shouldn't be explicit

Summary:
Containers should not tag their default constructor explicit,
because it means that you can't return an empty one with "return {};".
This diff removes the explicit tag from small_vector's default
constructor.

Reviewed By: meyering

Differential Revision: D5602749

fbshipit-source-id: 2a4a356134c69d01e3b040719dc6ffca0fe97e75

folly/small_vector.h

@@ -401,7 +401,7 @@ class small_vector
   typedef std::reverse_iterator<iterator>       reverse_iterator;
   typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
 
-  explicit small_vector() = default;
+  small_vector() = default;
 
   small_vector(small_vector const& o) {
     auto n = o.size();