clean up compiler warnings

did a test compile with clang and got rid of some warnings that have
been annoying me for a while:
vp8/decoder/detokenize.c: In function 'vp8_init_detokenizer':
vp8/decoder/detokenize.c:121: warning: assignment discards qualifiers from pointer target type
vp8/decoder/detokenize.c:122: warning: assignment discards qualifiers from pointer target type
vp8/decoder/detokenize.c:123: warning: assignment from incompatible pointer type
vp8/decoder/detokenize.c:124: warning: assignment discards qualifiers from pointer target type
vp8/decoder/detokenize.c:125: warning: assignment discards qualifiers from pointer target type
vp8/decoder/detokenize.c:128: warning: assignment discards qualifiers from pointer target type
vp8/decoder/detokenize.c:129: warning: assignment discards qualifiers from pointer target type
vp8/decoder/detokenize.c:130: warning: assignment discards qualifiers from pointer target type
vp8/decoder/detokenize.c:131: warning: assignment discards qualifiers from pointer target type

Change-Id: I78ddab176fe47cbeed30379709dc7bab01c0c2e4