clover: Add explicit virtual destructor to argument class

It is needed to destroy the v vector in scalar_argument
Fixes memory leaks on parameter set/bind.

v2: Drop redundant sclara_argument destructor

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/gallium/state_trackers/clover/core/kernel.hpp

@@ -93,6 +93,7 @@ namespace clover {
          /// Free any resources that were allocated in bind().
          virtual void unbind(exec_context &ctx) = 0;
 
+         virtual ~argument() {};
       protected:
          argument();