psnr.c: unsigned to signed
psnr.c: unsigned to signed

Since diff can be negative and sum of diff can be negative too.

Change-Id: I822a4e90cc4fa06c796b7a87a776cf62a23a7621

mbebenita/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,luctrudeau/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom