x86inc: put basicnop under ifdef to prevent compile failure.

This should fix the NASM box.
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/x86/x86inc.asm

@@ -628,9 +628,11 @@ SECTION .note.GNU-stack noalloc noexec nowrite progbits
         %elifidn %1, sse3
             %define movu lddqu
         %endif
+        %ifdef __YASM_VER__
         %if notcpuflag(mmx2)
             CPU basicnop
         %endif
+        %endif
     %else
         %xdefine SUFFIX
         %undef cpuname