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