virtio-mem: Fix build error due to improper use 'select'

As noted in:
https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt
"select should be used with care. select will force a symbol to a
value without visiting the dependencies."
Config VIRTIO_MEM should not select CONTIG_ALLOC directly.
Otherwise it will cause an error:
https://bugzilla.kernel.org/show_bug.cgi?id=208245

Signed-off-by: Weilong Chen <chenweilong@huawei.com>
Link: https://lore.kernel.org/r/20200619080333.194753-1-chenweilong@huawei.com
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>

drivers/virtio/Kconfig

@@ -85,7 +85,7 @@ config VIRTIO_MEM
 	depends on VIRTIO
 	depends on MEMORY_HOTPLUG_SPARSE
 	depends on MEMORY_HOTREMOVE
-	select CONTIG_ALLOC
+	depends on CONTIG_ALLOC
 	help
 	 This driver provides access to virtio-mem paravirtualized memory
 	 devices, allowing to hotplug and hotunplug memory.