Define the size_t type since readpassphrase(3) requires it in its

definition and sys/types.h is not listed within the synopsis of the man
page.

MFC after:	1 week

include/readpassphrase.h

@@ -39,6 +39,12 @@
 #define RPP_SEVENBIT    0x10		/* Strip the high bit from input. */
 
 #include <sys/cdefs.h>
+#include <sys/_types.h>
+
+#ifndef _SIZE_T_DECLARED
+typedef	__size_t	size_t;
+#define	_SIZE_T_DECLARED
+#endif
 
 __BEGIN_DECLS
 char * readpassphrase(const char *, char *, size_t, int);