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.

windows/wincons.c

@@ -335,7 +335,7 @@ void pgp_fingerprints(void)
 	  "  " PGP_PREV_MASTER_KEY_FP "\n", stdout);
 }
 
-void console_provide_logctx(void *logctx)
+void console_provide_logctx(LogContext *logctx)
 {
     console_logctx = logctx;
 }