fixes SSE2/AddNoiseTest.CheckCvsAssembly/0 with -funsigned-char.
visibly broken since:
0dc69c70 postproc : fix function parameters for noise functions.
where the types diverged (char vs. int8)
but likely the return changed in:
2ca24b00 postproc - move filling of noise buffer to vpx_dsp.
when multiple implementations were merged.

Change-Id: I176ca1f170217f05ba7872b0c4de63e41949e999