fix checking boolean variable which might contain a string (#165)

Author: dirk-thomas

rmw/cmake/get_rmw_typesupport.cmake

@@ -37,7 +37,7 @@ function(get_rmw_typesupport var rmw_implementation)
     set(resource_type "${resource_type}_${ARG_LANGUAGE}")
   endif()
   ament_index_has_resource(has_resource "${resource_type}" "${rmw_implementation}")
-  if(${has_resource})
+  if(has_resource)
     ament_index_get_resource(resource "${resource_type}" "${rmw_implementation}")
   else()
     set(resource "")