readline_buffer: add a couple const

contrib/epee/include/readline_buffer.h

@@ -13,11 +13,11 @@ namespace rdln
     void start();
     void stop();
     int process();
-    bool is_running()
+    bool is_running() const
     {
       return m_cout_buf != NULL;
     }
-    void get_line(std::string& line);
+    void get_line(std::string& line) const;
     void set_prompt(const std::string& prompt);
     
   protected:

contrib/epee/src/readline_buffer.cpp

@@ -62,7 +62,7 @@ void rdln::readline_buffer::stop()
   remove_line_handler();
 }
 
-void rdln::readline_buffer::get_line(std::string& line)
+void rdln::readline_buffer::get_line(std::string& line) const
 {
   std::unique_lock<std::mutex> lock(line_mutex);
   have_line.wait(lock);