rgw_main.cc: add missing virtual destructor for RGWRequest

CID 1160858 (#1 of 1): Non-virtual destructor (VIRTUAL_DTOR)
 nonvirtual_dtor: Class RGWLoadGenRequest has a destructor
 and a pointer to it is upcast to class RGWRequest which doesn't
 have a virtual destructor.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>