Make 'LogContext' a typedef visible throughout the code.

Same principle again - the more of these structures have globally
visible tags (even if the structure contents are still opaque in most
places), the fewer of them I can mistake for each other.

unix/gtkwin.c

@@ -162,7 +162,7 @@ struct gui_data {
     Backend *back;
     void *backhandle;
     Terminal *term;
-    void *logctx;
+    LogContext *logctx;
     int exited;
     struct unicode_data ucsdata;
     Conf *conf;