ring: fix declaration after statement

ring: fix declaration after statement

On gcc 5.4.0 / native aarch64 from Ubuntu 16.04:

In function '__rte_ring_move_prod_head':
rte_ring_c11_mem.h:69:3: 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_c11_mem.h:136:3: warning:
ISO C90 forbids mixed declarations and code
[-Wdeclaration-after-statement]
   const uint32_t prod_tail = r->prod.tail;
   ^

Fixes: 39368ebfc6 ("ring: introduce C11 memory model barrier option")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>