require --enable-psnr to build ssim

ssim.c comiles in a huge (512M) amount of global scratch space. Allocating
this data on the heap would be a better solution, but this file doesn't
need to be built at all in most cases, so as a first pass, disable it
except when doing opsnr.stt output (--enable-psnr).

Change-Id: I320d812f6d652a12516a16b52295ebff20b5bd42