libc: improve include usage for exterror sources

Include sys/types.h by exterr.h, since size_t is used.
Drop include of sys/exterr_cat.h, it is useless for the only prototype
provided.

Reviewed by:	mckusick
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D55337

include/exterr.h

@@ -11,8 +11,7 @@
 #ifndef _EXTERR_H_
 #define	_EXTERR_H_
 
-#include <sys/cdefs.h>
-#include <sys/exterr_cat.h>
+#include <sys/types.h>
 
 __BEGIN_DECLS
 int uexterr_gettext(char *buf, size_t bufsz);

lib/libc/gen/uexterr_format.c

@@ -9,6 +9,7 @@
  */
 
 #include <sys/param.h>
+#include <sys/exterr_cat.h>
 #include <sys/exterrvar.h>
 #include <exterr.h>
 #include <stdbool.h>