ring: fix build with icc

ring: fix build with icc

build error:
In file included from .../lib/librte_ring/rte_ring.c(90):
.../lib/librte_ring/rte_ring.h(162):
error #1366: a reduction in alignment without the "packed" attribute
is ignored
  } __rte_cache_aligned;
      ^

Alignment attribute moved to first element of the struct

Fixes: a6619414e0a9 ("ring: make struct and macros type agnostic")
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>