Ignore the internal mouse on the Olivetti machine if the mouse is not set to internal, fixes serial mouse on both the Olivetti M24 and M240, closes #2880.

src/machine/m_xt_olivetti.c

@@ -1385,10 +1385,12 @@ m24_kbd_init(m24_kbd_t *kbd)
     m24_kbd_reset(kbd);
     timer_add(&kbd->send_delay_timer, m24_kbd_poll, kbd, 1);
 
+    if (mouse_type == MOUSE_TYPE_INTERNAL) {
         /* Tell mouse driver about our internal mouse. */
         mouse_reset();
         mouse_set_buttons(2);
         mouse_set_poll(ms_poll, kbd);
+    }
 
     keyboard_set_table((kbd->id == 0x01) ? scancode_olivetti_m24_deluxe : scancode_olivetti_m240);
     keyboard_set_is_amstrad(0);