Fix build with --disable-mdct

Originally committed as revision 21764 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/fft.c

@@ -99,9 +99,11 @@ av_cold int ff_fft_init(FFTContext *s, int nbits, int inverse)
 
     s->fft_permute = ff_fft_permute_c;
     s->fft_calc    = ff_fft_calc_c;
+#if CONFIG_MDCT
     s->imdct_calc  = ff_imdct_calc_c;
     s->imdct_half  = ff_imdct_half_c;
     s->mdct_calc   = ff_mdct_calc_c;
+#endif
     s->exptab1     = NULL;
     s->split_radix = 1;