aacsbr: Make dk signed. There is no point in it being unsigned.

Originally committed as revision 23671 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/aacsbr.c

@@ -379,8 +379,7 @@ static int sbr_make_f_master(AACContext *ac, SpectralBandReplication *sbr,
     }
 
     if (!spectrum->bs_freq_scale) {
-        unsigned int dk;
-        int k2diff;
+        int dk, k2diff;
 
         dk = spectrum->bs_alter_scale + 1;
         sbr->n_master = ((sbr->k[2] - sbr->k[0] + (dk&2)) >> dk) << 1;