ring: fix C++ cast error

ring: fix C++ cast error

build error:
include/rte_ring.h:459:22: error: invalid conversion from ‘void*’
to ‘void**’ [-fpermissive]
  ENQUEUE_PTRS(r, &r[1], prod_head, obj_table, n, void *);

Implicit casts of void* to void** are considered warnings in some
compilers.  E.g. g++ version 5.8.  Cast directly to object types

Fixes: a6619414 ("ring: make struct and macros type agnostic")
Signed-off-by: Ed Czeck <ed.czeck@atomicrules.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>