removed sign mismatch warning

Change-Id: Iaa40b472f6c1c48bb3bb47332b6fcf36d7f3c10e

vp9/common/vp9_common.h

@@ -49,7 +49,7 @@ extern "C" {
 #define vp9_zero_array(dest, n) vpx_memset(dest, 0, n * sizeof(*dest))
 
 static INLINE uint8_t clip_pixel(int val) {
-  return (val > 255) ? 255u : (val < 0) ? 0u : val;
+  return (val > 255) ? 255 : (val < 0) ? 0 : val;
 }
 
 static INLINE int clamp(int value, int low, int high) {