Set usage() to show -d flag

mkesdb supports the -d flag for enabling debug mode, as documented in the manual.

PR:		209865
Submitted by:	Maya Rashish <coypu AT sdf DOT org>
Reviewed by:	imp
Approved by:	bcr (mentor)
Obtained from:	NetBSD
MFC after:	7 days
Differential Revision:	https://reviews.freebsd.org/D11987

usr.bin/mkesdb/yacc.y

@@ -278,8 +278,8 @@ usage(void)
 {
 	errx(EXIT_FAILURE,
 	    "usage:\n"
-	    "\t%s [-o outfile] [infile]\n"
-	    "\t%s -m [-o outfile] [infile]",
+	    "\t%s [-d] [-o outfile] [infile]\n"
+	    "\t%s -m [-d] [-o outfile] [infile]",
 	    getprogname(), getprogname());
 }