ring: remove useless variables

ring: remove useless variables

There were warnings with GCC 8.1:

In function '__rte_ring_move_prod_head':
rte_ring_generic.h:76:3:
warning: ISO C90 forbids mixed declarations and code
[-Wdeclaration-after-statement]
   const uint32_t cons_tail = r->cons.tail;
   ^~~~~

In function '__rte_ring_move_cons_head':
rte_ring_generic.h:147:3:
warning: ISO C90 forbids mixed declarations and code
[-Wdeclaration-after-statement]
   const uint32_t prod_tail = r->prod.tail;

Fixes: 0dfc98c507b1 ("ring: separate out head index manipulation")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>