normalize WebPValidatePicture declaration w/definition
quiets a warning under visual studio:
src\enc\picture_enc.c(48) : warning C4028: formal parameter 1 different
from declaration
@@ -494,7 +494,7 @@ int VP8FilterStrengthFromDelta(int sharpness, int delta);
// Returns true if 'picture' is non-NULL and dimensions/colorspace are within
// their valid ranges. If returning false, the 'error_code' in 'picture' is
// updated.
-int WebPValidatePicture(const WebPPicture* picture);
+int WebPValidatePicture(const WebPPicture* const picture);
// Remove reference to the ARGB/YUVA buffer (doesn't free anything).
void WebPPictureResetBuffers(WebPPicture* const picture);
