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

vp8/vp8cx.mk

diff --git a/vp8/vp8cx.mk b/vp8/vp8cx.mk
index 544a3b3..9496ef0 100644
--- a/vp8/vp8cx.mk
+++ b/vp8/vp8cx.mk
@@ -74,7 +74,7 @@
 VP8_CX_SRCS-yes += encoder/ratectrl.c
 VP8_CX_SRCS-yes += encoder/rdopt.c
 VP8_CX_SRCS-yes += encoder/sad_c.c
-VP8_CX_SRCS-yes += encoder/ssim.c
+VP8_CX_SRCS-$(CONFIG_PSNR) += encoder/ssim.c
 VP8_CX_SRCS-yes += encoder/tokenize.c
 VP8_CX_SRCS-yes += encoder/treewriter.c
 VP8_CX_SRCS-yes += encoder/variance_c.c