[CMake][Bug] Fix use of lua sources cmake variable.

src/CMakeLists.txt

@@ -56,7 +56,7 @@ IF(LUA5_1_FOUND)
       multibody/parser/lua/lua_tables.cpp
       multibody/parser/lua.cpp
     )
-  LIST(APPEND ${PROJECT_NAME}_SOURCES ${PROJECT_NAME}_MULTIBODY_PARSER_LUA_SOURCES)
+  LIST(APPEND ${PROJECT_NAME}_SOURCES ${${PROJECT_NAME}_MULTIBODY_PARSER_LUA_SOURCES})
 ENDIF(LUA5_1_FOUND)