add absolute path

add absolute path to profile picture path, so we can execute script outside src folder.

It raise error if I execute wa script that located outside src folder.

```bash
string(18) "pictures/78219.jpg"
PHP Warning:  copy(pictures/78219.jpg): failed to open stream: No such file or directory in /home/xxx/project/whatsapp/php/src/whatsprot.class.php on line 2927
```