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>