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>

samples/vfio-mdev/Makefile

@@ -1,13 +1 @@
-#
-# Makefile for mtty.c file
-#
-KERNEL_DIR:=/lib/modules/$(shell uname -r)/build
-
-obj-m:=mtty.o
-
-modules clean modules_install:
-	$(MAKE) -C $(KERNEL_DIR) SUBDIRS=$(PWD) $@
-
-default: modules
-
-module: modules
+obj-$(CONFIG_SAMPLE_VFIO_MDEV_MTTY) += mtty.o