AP_Scripting: ignore hidden Lua files

On macOS, sometimes ._script.lua is created to store metadata when the
user copies script.lua over to their SD card. Previously, the scripting
engine would barf since the file is not Lua. Now, these files are
ignored.

Also avoids a case where a hidden and valid script might be loaded
without the user's knowledge.

libraries/AP_Scripting/lua_scripts.cpp

@@ -268,8 +268,8 @@ void lua_scripts::load_all_scripts_in_dir(lua_State *L, const char *dirname) {
             continue;
         }
 
-        if (strncmp(&de->d_name[length-4], ".lua", 4)) {
-            // doesn't end in .lua
+        if ((de->d_name[0] == '.') || strncmp(&de->d_name[length-4], ".lua", 4)) {
+            // starts with . (hidden file) or doesn't end in .lua
             continue;
         }