x86/dsputilenc: add missing guards to ff_pix_sum16_xop

XOP support was added in Yasm 1.0.0 and Nasm 2.06, and we still
support older versions.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>