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function prototype with unspecified argument is obsolete since c99.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/keymap.c

@@ -47,7 +47,7 @@ get_layout(char *syms, int grp_num)
 }
 
 const char *
-keymap(void)
+keymap(const char *unused)
 {
 	Display *dpy;
 	XkbDescRec *desc;