clangcl build fix for __cpuid in psnr util. Since clangcl provides the intrinsic thru its Visual C emulation, don't duplicately define the function with an inline version, which is normally needed for gcc/clang.
BUG=412
TESTED=set GYP_DEFINES=clang=1 & gyp_libyuv -fninja libyuv_test.gyp
R=brucedawson@google.com

Review URL: https://webrtc-codereview.appspot.com/40339004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1331 16f28f9a-4ce2-e073-06de-1de4eb20be90