ifconfig: set boolean to true rather than incrementing

GCC warns about "increment of a boolean expression."

Reported by:	amd64-gcc12 Cirrus-CI run
Sponsored by:	The FreeBSD Foundation

sbin/ifconfig/ifconfig.c

@@ -501,7 +501,7 @@ args_parse(struct ifconfig_args *args, int argc, char *argv[])
 			args->printkeys = true;
 			break;
 		case 'l':	/* scan interface names only */
-			args->namesonly++;
+			args->namesonly = true;
 			break;
 		case 'm':	/* show media choices in status */
 			args->supmedia = true;