lpm: fix scalar version header for C++

lpm: fix scalar version 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.

The generated assembly is identical to the code without the fix (checked
both on x86 and RISC-V).

Fixes: 406937f89ffd ("lpm: add scalar version of lookupx4")
Signed-off-by: Stanislaw Kardach <kda@semihalf.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>