Fixed #307.

Added 'inline' keyword to ext's constructor that defined at outside of the class definition.

Author: redboltz

include/msgpack/adaptor/ext.hpp

@@ -183,7 +183,7 @@ class ext_ref {
     friend struct msgpack::adaptor::object<msgpack::type::ext_ref>;
 };
 
-ext::ext(ext_ref const& x) {
+inline ext::ext(ext_ref const& x) {
     // size limit has aleady been checked at ext_ref's constructor
     m_data.reserve(x.size() + 1);