Fix one gcc compiler warning

../libvpx/vp8/encoder/bitstream.c: In function ‘pack_inter_mode_mvs’:
../libvpx/vp8/encoder/bitstream.c:1026: warning: array subscript has type ‘char’

Change-Id: Ic77491e0a172fa1821e5b3e914d0dc41fe87c00f

vp8/common/blockd.h

@@ -169,7 +169,7 @@ typedef struct
         MV  as_mv;
     } mv;
 
-    char partitioning;
+    unsigned char partitioning;
     unsigned char mb_skip_coeff;                                //does this mb has coefficients at all, 1=no coefficients, 0=need decode tokens
     unsigned char dc_diff;
     unsigned char need_to_clamp_mvs;