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/os/mail/mail_encoding.h

@@ -6,7 +6,6 @@
 */
 
 
-#include <size_t.h>
 #include <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"

src/add-ons/kernel/file_systems/shared/crc32.cpp

@@ -49,7 +49,7 @@
 #ifdef __HAIKU__
 #include <endian.h>
 #include <kernel.h>
-#include <size_t.h>
+#include <stddef.h>
 #include <stdint.h>
 uint32 calculate_crc32c(uint32 crc32c, const unsigned char *buffer,
 	unsigned int length);