ITS#10011 servers: fix -Wstrict-prototypes

For more information, see LWN.net [0] or LLVM's Discourse [1], the Gentoo wiki [2],
or the (new) c-std-porting mailing list [3].

[0] https://lwn.net/Articles/913505/
[1] https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213
[2] https://wiki.gentoo.org/wiki/Modern_C_porting
[3] hosted at lists.linux.dev.

Signed-off-by: Sam James <sam@gentoo.org>

servers/slapd/syslog.c

@@ -209,7 +209,7 @@ openlog(const char *ident, int logstat, int logfac)
 }
 
 void
-closelog()
+closelog(void)
 {
 	(void)close(LogFile);
 	LogFile = -1;