Enforce strict prototypes

i3 will now compile with no warnings when -Wstrict-prototypes is used.

include/configuration.h

@@ -427,7 +427,7 @@ void ungrab_all_keys(xcb_connection_t *conn);
  * Sends the current bar configuration as an event to all barconfig_update listeners.
  *
  */
-void update_barconfig();
+void update_barconfig(void);
 
 /**
  * Kills the configerror i3-nagbar process, if any.

include/libi3.h

@@ -472,7 +472,7 @@ xcb_visualtype_t *get_visualtype(xcb_screen_t *screen);
  * release version), based on the git version number.
  *
  */
-bool is_debug_build() __attribute__((const));
+bool is_debug_build(void) __attribute__((const));
 
 /**
  * Returns the name of a temporary file with the specified prefix.