Use (void) instead of () for functions without args (Thanks fernandotcl)

See also:
http://article.gmane.org/gmane.linux.kernel/1268792

The C compiler will handle (void) as "no arguments" and () as "variadic
function" (equivalent to (...)) which might lead to subtle errors, such
as the one which was fixed with commit 0ea64ae4.

include/randr.h

@@ -61,13 +61,13 @@ void init_ws_for_output(Output *output, Con *content);
  * (Re-)queries the outputs via RandR and stores them in the list of outputs.
  *
  */
-void randr_query_outputs();
+void randr_query_outputs(void);
 
 /**
  * Returns the first output which is active.
  *
  */
-Output *get_first_output();
+Output *get_first_output(void);
 
 /**
  * Returns the output with the given name if it is active (!) or NULL.