x86inc: try to put amdnop under ifdef to prevent compile failure

based on similar amdnop usage in ffmpeg
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/x86/x86inc.asm

@@ -604,7 +604,9 @@ SECTION .note.GNU-stack noalloc noexec nowrite progbits
 ; All subsequent functions (up to the next INIT_CPUFLAGS) is built for the specified cpu.
 ; You shouldn't need to invoke this macro directly, it's a subroutine for INIT_MMX &co.
 %macro INIT_CPUFLAGS 0-2
+%ifdef __YASM_VER__
     CPU amdnop
+%endif
     %if %0 >= 1
         %xdefine cpuname %1
         %assign cpuflags cpuflags_%1