FDC workaround

The Internal FDC is added when needed for AT machines too.
Some speculative "work" on the main FDC code

src/machine/m_xt_laserxt.c

@@ -16,6 +16,7 @@
 #include <86box/timer.h>
 #include <86box/fdd.h>
 #include <86box/fdc.h>
+#include <86box/fdc_ext.h>
 #include <86box/gameport.h>
 #include <86box/keyboard.h>
 
@@ -169,6 +170,8 @@ machine_xt_lxt3_init(const machine_t *model)
     pit_ctr_set_out_func(&pit->counters[1], pit_refresh_timer_xt);
 
     device_add(&keyboard_xt_lxt3_device);
+
+    if (fdc_type == FDC_INTERNAL)
     device_add(&fdc_xt_device);
     nmi_init();
     if (joystick_type)