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>

libavcodec/x86/dsputilenc.asm

@@ -500,8 +500,10 @@ INIT_MMX mmx
 PIX_SUM16 0, 16, 3, 0
 INIT_XMM sse2
 PIX_SUM16 6, 8,  3, 2
+%if HAVE_XOP_EXTERNAL
 INIT_XMM xop
 PIX_SUM16 5, 4,  4, 4
+%endif
 
 ; int ff_pix_norm1_mmx(uint8_t *pix, int line_size)
 ; %1 = number of xmm registers used