Make UnpackerErrCategory an unique category

This is a problem in Release compilation where the
theUnpackerErrCategory variable is optimized and created on each
compilation unit, and then the error_handling.cpp test fails in:

  REQUIRE(ec == msgpack::UnpackerError::OutOfRange);

Because the category is different on each compilation unit.

msgpack/include/msgpack/msgpack.hpp

@@ -35,12 +35,12 @@ struct UnpackerErrCategory : public std::error_category {
         return "(unrecognized error)";
     }
   };
-};
 
-const UnpackerErrCategory theUnpackerErrCategory{};
+};
 
 inline
 std::error_code make_error_code(msgpack::UnpackerError e) {
+  static UnpackerErrCategory theUnpackerErrCategory;
   return {static_cast<int>(e), theUnpackerErrCategory};
 }
 }