veriexec(4): Fix a compiler error

When building WITH_BEARSSL=1 veriexec(4) failes to compile.
So update the function prototype so that veriexec(4)
compiles again.

Reported by:	gbe
Reviewed by:	mjg, gbe
Approved by:	mjg
Pull Request:	https://github.com/freebsd/freebsd-src/pull/657

sbin/veriexec/veriexec.c

@@ -50,7 +50,7 @@ const char *Cdir = NULL;
  * @return always returns code 0
  */
 static int
-veriexec_usage()
+veriexec_usage(void)
 {
 	printf("%s",
 	    "Usage:\tveriexec [-h] [-i state] [-C] [-xv state|verbosity] [path]\n");