[pfctl_tests] Add missing void to empty function declaration

Our gcc-6.4 flags require non-empty function declarations.
Fix this to match the rest of the codebase.

Tested:

* compiled on gcc-6.4 for amd64

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D26795

sbin/pfctl/tests/pfctl_test.c

@@ -68,7 +68,7 @@ __FBSDID("$FreeBSD$");
  */
 
 static bool
-check_pf_module_available()
+check_pf_module_available(void)
 {
 	int modid;
 	struct module_stat stat;