include <stddef.h> so that ptrdiff_t is again available

include/libbb.h

@@ -23,6 +23,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>
+#include <stddef.h>
 #include <string.h>
 #include <strings.h>
 #include <sys/ioctl.h>