eal/riscv: fix vector header for C++

eal/riscv: fix vector header for C++

rte_xmm_t is a union type which wraps around xmm_t and maps its contents
to scalar structures. Since C++ has stricter type conversion rules than
C, the rte_xmm_t::x has to be used instead of C-casting.

Fixes: f22e705ebf12 ("eal/riscv: support RISC-V architecture")
Signed-off-by: Stanislaw Kardach <kda@semihalf.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>