Add wchar.h to compat.h, from Koichi Shiraishi.

TODO

@@ -89,7 +89,8 @@
 	  window->session->global, and for session we look session->global.
 	  problem: what about windows in multiple sessions? there are contexts
 	  where we do not know which session, or where multiple choices makes
-          no sense
+	  no sense... could at least have one global list for all types of
+	  global options and keep separate window,session lists
 	* the way pane, window, session destroy is handled is too complicated
 	  and the distinction between session.c, window.c and server-fn.c
 	  functions is not clear. could we just have kill_pane(),

compat.h

@@ -22,6 +22,7 @@
 
 #include <limits.h>
 #include <stdio.h>
+#include <wchar.h>
 
 #ifndef __GNUC__
 #define __attribute__(a)