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/x.h

@@ -108,7 +108,7 @@ void x_set_name(Con *con, const char *name);
  * Sets up i3 specific atoms (I3_SOCKET_PATH and I3_CONFIG_PATH)
  *
  */
-void x_set_i3_atoms();
+void x_set_i3_atoms(void);
 
 /**
  * Set warp_to coordinates.  This will trigger on the next call to