AP_Scripting: Add missing const in member functions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

libraries/AP_Scripting/lua_repl.cpp

@@ -44,7 +44,7 @@ static int msghandler(lua_State *L) {
 ** Interface to 'lua_pcall', which sets appropriate message function
 ** and C-signal handler. Used to run all chunks.
 */
-int lua_scripts::docall(lua_State *L, int narg, int nres) {
+int lua_scripts::docall(lua_State *L, int narg, int nres) const {
     int status;
     int base = lua_gettop(L) - narg;  /* function index */
     lua_rawgeti(L, LUA_REGISTRYINDEX, sandbox_ref);

libraries/AP_Scripting/lua_scripts.h

@@ -99,7 +99,7 @@ private:
     int pushline(lua_State *L, int firstline);
     int incomplete(lua_State *L, int status);
     const char * get_prompt(lua_State *L, int firstline);
-    int docall(lua_State *L, int narg, int nres);
+    int docall(lua_State *L, int narg, int nres) const;
     int sandbox_ref;
 
     script_info *scripts; // linked list of scripts to be run, sorted by next run time (soonest first)