Move declaration of write_aclip into winstuff.h.

It's actually a function specific to the Windows front end, and has
been all along - but I've only just noticed that no other front end
either uses it or defines it.

windows/winstuff.h

@@ -242,6 +242,13 @@ void quit_help(HWND hwnd);
 GLOBAL Terminal *term;
 GLOBAL void *logctx;
 
+/*
+ * Windows-specific clipboard helper function shared with windlg.c,
+ * which takes the data string in the system code page instead of
+ * Unicode.
+ */
+void write_aclip(void *frontend, char *, int, int);
+
 #define WM_NETEVENT  (WM_APP + 5)
 
 /*