add a few missing <stddef.h> includes for NULL

and remove unused includes in sharpyuv/

Change-Id: If10538a994bd5dc55126f1485f2b163933ad8e91

sharpyuv/sharpyuv.c

@@ -15,7 +15,7 @@
 
 #include <assert.h>
 #include <limits.h>
-#include <math.h>
+#include <stddef.h>
 #include <stdlib.h>
 #include <string.h>
 

sharpyuv/sharpyuv_csp.c

@@ -13,7 +13,7 @@
 
 #include <assert.h>
 #include <math.h>
-#include <string.h>
+#include <stddef.h>
 
 static int ToFixed16(float f) { return (int)floor(f * (1 << 16) + 0.5f); }
 

src/dsp/cpu.h

@@ -14,6 +14,8 @@
 #ifndef WEBP_DSP_CPU_H_
 #define WEBP_DSP_CPU_H_
 
+#include <stddef.h>
+
 #ifdef HAVE_CONFIG_H
 #include "src/webp/config.h"
 #endif