do not rely on obsolete feature

function prototype with unspecified argument is obsolete since c99.

additionally some of these function which don't take any argument were
being called with a `const char *` arg, which is UB.

fix both these issues by declararing ALL the components to accept a
`const char *`, and name the arg "unused" if it's meant to be ignored.

components/user.c

@@ -8,13 +8,13 @@
 #include "../slstatus.h"
 
 const char *
-gid(void)
+gid(const char *unused)
 {
 	return bprintf("%d", getgid());
 }
 
 const char *
-username(void)
+username(const char *unused)
 {
 	struct passwd *pw;
 
@@ -27,7 +27,7 @@ username(void)
 }
 
 const char *
-uid(void)
+uid(const char *unused)
 {
 	return bprintf("%d", geteuid());
 }