add stddef.h for ptrdiff_t

thirdparty/SZ/sz/src/sz_double.c

@@ -10,6 +10,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <stddef.h>
 #include <string.h>
 #include <unistd.h>
 #include <math.h>

thirdparty/SZ/sz/src/sz_float.c

@@ -10,6 +10,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <stddef.h>
 #include <string.h>
 #include <unistd.h>
 #include <math.h>