eal/ppc: fix build

Compilation is broken on ppc:

  CC otx2_rx.o
In file included from .../drivers/net/octeontx2/otx2_rx.c:5:0:
.../builds/ppc_64-power8-linux-gcc/include/rte_vect.h:29:17:
error: expected declaration specifiers or ‘...’ before numeric constant
 } __rte_aligned(16) rte_xmm_t;
                 ^~
compilation terminated due to -Wfatal-errors.

Fixes: f35e5b3e07b2 ("replace alignment attributes")

Signed-off-by: David Marchand <david.marchand@redhat.com>
Tested-by: Thomas Monjalon <thomas@monjalon.net>