warning: pointer targets differ in signedness

vp8/encoder/rdopt.c:728: warning: pointer targets in passing argument 3
of 'macro_block_yrd' differ in signedness
vp8/encoder/rdopt.c:541: note: expected 'int *' but argument is of type
'unsigned int *'

distortion is signed when calling macro_block_yrd is both other cases,
as well as for RDCOST

Change-Id: I5e22358b7da76a116f498793253aac8099cb3461