Flush stdout when using -print

This ensures writes happen on every new notification when output is
redirected to a file.

Closes #794

src/notification.c

@@ -98,6 +98,7 @@ void notification_print(const struct notification *n)
                 printf("\n");
         }
         printf("}\n");
+        fflush(stdout);
 }
 
 /* see notification.h */