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/workspace.h

@@ -69,31 +69,31 @@ void workspace_show_by_name(const char *num);
  * Returns the next workspace.
  *
  */
-Con* workspace_next();
+Con* workspace_next(void);
 
 /**
  * Returns the previous workspace.
  *
  */
-Con* workspace_prev();
+Con* workspace_prev(void);
 
 /**
  * Returns the next workspace on the same output
  *
  */
-Con* workspace_next_on_output();
+Con* workspace_next_on_output(void);
 
 /**
  * Returns the previous workspace on the same output
  *
  */
-Con* workspace_prev_on_output();
+Con* workspace_prev_on_output(void);
 
 /**
  * Focuses the previously focused workspace.
  *
  */
-void workspace_back_and_forth();
+void workspace_back_and_forth(void);
 
 
 #if 0