Drop direct includes of size_t.h.

It's a non-standard header we inherited from BeOS. According to the
POSIX specification, size_t should be declared from including stddef.h
and/or sys/types.h, and ssize_t from sys/types.h.

headers/private/debugger/user_interface/util/UiUtils.h

@@ -6,8 +6,7 @@
 #ifndef UI_UTILS_H
 #define UI_UTILS_H
 
-#include <size_t.h>
-
+#include <stddef.h>
 #include <image.h>
 
 #include "Types.h"