Change const to constexpr so that symbols are resolved (#442)

Author: jplotzke

src/well_known_messages.h

@@ -49,9 +49,9 @@ namespace grpc_labview
             virtual ~I2DArray() = default;
 
         protected:
-            static const int _rowsIndex = 1;
-            static const int _columnsIndex = 2;
-            static const int _dataIndex = 3;
+            static constexpr int _rowsIndex = 1;
+            static constexpr int _columnsIndex = 2;
+            static constexpr int _dataIndex = 3;
         };
 
         //---------------------------------------------------------------------
@@ -223,4 +223,4 @@ namespace grpc_labview
             void CopyArrayFromClusterToMessage(int totalElements, LV2DArrayHandle array, const std::shared_ptr<LVMessageValue>& dataFieldValue) override;
         };
     }
-}
+}