vfio-mdev: Fix mtty sample driver building

This sample driver was originally under Documentation/ and was moved
to samples, but build support was never adjusted for the new location.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Kirti Wankhede <kwankhede@nvidia.com>
diff --git a/samples/Kconfig b/samples/Kconfig
index a6d2a43..b124f62 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -105,4 +105,11 @@
 	help
 	  Build samples of blackfin gptimers sample module.
 
+config SAMPLE_VFIO_MDEV_MTTY
+	tristate "Build VFIO mtty example mediated device sample code -- loadable modules only"
+	depends on VFIO_MDEV_DEVICE && m
+	help
+	  Build a virtual tty sample driver for use as a VFIO
+	  mediated device
+
 endif # SAMPLES