avcodec/x86/mpegaudiodsp: silence -Wunused-variable on --disable-mmx

This silences -Wunused-variable when compiled with --disable-mmx, e.g
http://fate.ffmpeg.org/log.cgi?time=20150919094617&log=compile&slot=x86_64-archlinux-gcc-disable-mmx.
The alternative of header guards will make it far too ugly.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/x86/mpegaudiodsp.c

@@ -241,7 +241,7 @@ DECL_IMDCT_BLOCKS(avx,avx)
 
 av_cold void ff_mpadsp_init_x86(MPADSPContext *s)
 {
-    int cpu_flags = av_get_cpu_flags();
+    av_unused int cpu_flags = av_get_cpu_flags();
 
     int i, j;
     for (j = 0; j < 4; j++) {