Resolve default constructor error in Xcode 7.2.1 and 8.2.1

Fix issue that produces the error 'default initialization of an object
of const type 'const detail::buffer_size' without a user-provided
default constructor'

include/fmt/os.h

@@ -394,7 +394,7 @@ struct ostream_params {
 
 FMT_END_DETAIL_NAMESPACE
 
-constexpr detail::buffer_size buffer_size;
+constexpr detail::buffer_size buffer_size{};
 
 /** A fast output stream which is not thread-safe. */
 class FMT_API ostream final : private detail::buffer<char> {