Output the PSNR for the entire file.

If --psnr option is enabled vpxenc will output PSNR values for the
entire file. Added a \n before final output to make sure the output
is on its own line. Overall and Avg psnr matches the values written
to opsnr.stt file.

Change-Id: Ibac5fa9baf8d5a626ea0d6ba161b484e6e8427ee