driver core: fix build for CONFIG_BLOCK not enabled

Fix build errors when CONFIG_BLOCK is not enabled:

drivers/base/core.c: In function 'get_device_parent':
drivers/base/core.c:634: error: 'block_class' undeclared (first use in this function)
drivers/base/core.c: In function 'device_add_class_symlinks':
drivers/base/core.c:723: error: 'block_class' undeclared (first use in this function)
drivers/base/core.c: In function 'device_remove_class_symlinks':
drivers/base/core.c:751: error: 'block_class' undeclared (first use in this function)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>