x86/xvididct: add some yasm guards

Should fix compilation on compilers with less-than-ideal dead code elimination
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/x86/xvididct_init.c

@@ -26,7 +26,7 @@
 #include "idctdsp.h"
 #include "xvididct.h"
 
-#if ARCH_X86_32
+#if ARCH_X86_32 && HAVE_YASM
 static void xvid_idct_mmx_put(uint8_t *dest, int line_size, short *block)
 {
     ff_xvid_idct_mmx(block);
@@ -55,6 +55,7 @@ static void xvid_idct_mmxext_add(uint8_t *dest, int line_size, short *block)
 av_cold void ff_xvid_idct_init_x86(IDCTDSPContext *c, AVCodecContext *avctx,
                                    unsigned high_bit_depth)
 {
+#if HAVE_YASM
     int cpu_flags = av_get_cpu_flags();
 
     if (high_bit_depth ||
@@ -84,4 +85,5 @@ av_cold void ff_xvid_idct_init_x86(IDCTDSPContext *c, AVCodecContext *avctx,
         c->idct      = ff_xvid_idct_sse2;
         c->perm_type = FF_IDCT_PERM_SSE2;
     }
+#endif /* HAVE_YASM */
 }