psnr tool, work around for ios 64 bit compiler where int passed into assembly…

psnr tool, work around for ios 64 bit compiler where int passed into assembly needs to be explicitely cast to 'w' register.
BUG=437
TESTED=untested
R=bcornell@google.com

Review URL: https://webrtc-codereview.appspot.com/53389004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1403 16f28f9a-4ce2-e073-06de-1de4eb20be90

util/psnr.cc

@@ -87,7 +87,7 @@ static uint32 SumSquareError_NEON(const uint8* src_a,
   "1:                                          \n"
     "ld1        {v0.16b}, [%0], #16            \n"
     "ld1        {v1.16b}, [%1], #16            \n"
-    "subs       %2, %2, #16                    \n"
+    "subs       %w2, %w2, #16                  \n"
     "usubl      v2.8h, v0.8b, v1.8b            \n"
     "usubl2     v3.8h, v0.16b, v1.16b          \n"
     "smlal      v16.4s, v2.4h, v2.4h           \n"