staging: vboxvideo: select GENERIC_ALLOCATOR

The vboxvideo code uses various gen_pool_* functions, so it needs
lib/genalloc.c to be built. In some configs this is not happening,
so add select GENERIC_ALLOCATOR to the Kconfig file to enforce this.

Note all other Kconfig references to GENERIC_ALLOCATOR also use select.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>