Add virtual destructor

It was causing enough warnings to trigger the Travis CI log overflow
limit...

include/CL/sycl/accessor/detail/accessor_base.hpp

@@ -63,6 +63,9 @@ class accessor_base {
 #ifdef TRISYCL_OPENCL
   /// Get the boost::compute::buffer or throw if unset
   virtual boost::compute::buffer get_cl_buffer() const = 0;
+
+  // Virtual to call the real destructor
+  virtual ~accessor_base() {}
 #endif
 
 };

include/CL/sycl/error_handler.hpp

@@ -42,7 +42,7 @@ struct error_handler {
   */
   static trisycl::default_error_handler default_handler;
 
-
+  // Virtual to call the real destructor
   virtual ~error_handler() = 0;
 };