style.9: Correct usage's definition to match its declaration

Suggested by:	emaste
Reviewed by:	delphij, imp, rgrimes, vangyzen (earlier version)
Sponsored by:	Dell EMC Isilon
Differential Revision:	(part of D20448)

share/man/man9/style.9

@@ -26,7 +26,7 @@
 .\"	From: @(#)style	1.14 (Berkeley) 4/28/95
 .\" $FreeBSD$
 .\"
-.Dd November 1, 2018
+.Dd May 28, 2019
 .Dt STYLE 9
 .Os
 .Sh NAME
@@ -777,7 +777,7 @@ vaf(const char *fmt, ...)
 }
 
 static void
-usage()
+usage(void)
 {
 	/* Insert an empty line if the function has no local variables. */
 .Ed