fastssim: Add some missing consts

Change-Id: Id36f180032c8a92c686da6f716a7468332b23b94

vpx_dsp/fastssim.c

diff --git a/vpx_dsp/fastssim.c b/vpx_dsp/fastssim.c
index 25f01e5..97dba33 100644
--- a/vpx_dsp/fastssim.c
+++ b/vpx_dsp/fastssim.c
@@ -444,7 +444,8 @@
   return 10 * (log10(_weight) - log10(_weight - _ssim));
 }
 
-double vpx_calc_fastssim(YV12_BUFFER_CONFIG *source, YV12_BUFFER_CONFIG *dest,
+double vpx_calc_fastssim(const YV12_BUFFER_CONFIG *source,
+                         const YV12_BUFFER_CONFIG *dest,
                          double *ssim_y, double *ssim_u, double *ssim_v) {
   double ssimv;
   vpx_clear_system_state();

vpx_dsp/ssim.h

diff --git a/vpx_dsp/ssim.h b/vpx_dsp/ssim.h
index d25e714..19f4df2 100644
--- a/vpx_dsp/ssim.h
+++ b/vpx_dsp/ssim.h
@@ -80,7 +80,8 @@
                       const YV12_BUFFER_CONFIG *dest,
                       double *ssim_y, double *ssim_u, double *ssim_v);
 
-double vpx_calc_fastssim(YV12_BUFFER_CONFIG *source, YV12_BUFFER_CONFIG *dest,
+double vpx_calc_fastssim(const YV12_BUFFER_CONFIG *source,
+                         const YV12_BUFFER_CONFIG *dest,
                          double *ssim_y, double *ssim_u, double *ssim_v);
 
 double vpx_psnrhvs(const YV12_BUFFER_CONFIG *source,