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/libi3.h

@@ -197,7 +197,7 @@ void set_font(i3Font *font);
  * Frees the resources taken by the current font.
  *
  */
-void free_font();
+void free_font(void);
 
 /**
  * Converts the given string to UTF-8 from UCS-2 big endian. The return value