mdmfs: Use standard bool definition

Include `<stdbool.h>` instead of defining a local bool enum.
This avoids duplicating a standard type name and keeps the
source compatible with headers that provide bool as a macro,
or in case of C23 that compilers provide it as keyword.

Signed-off-by:	Faraz Vahedi <kfv@kfv.io>
Reviewed by:	fuz
MFC after:	1 month
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2203

sbin/mdmfs/mdmfs.c

@@ -49,14 +49,13 @@
 #include <paths.h>
 #include <pwd.h>
 #include <stdarg.h>
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <ctype.h>
 #include <unistd.h>
 
-typedef enum { false, true } bool;
-
 struct mtpt_info {
 	uid_t		 mi_uid;
 	bool		 mi_have_uid;