staging: zcache: fix Kconfig crypto dependency

ZCACHE is a boolean in the Kconfig.  When selected, it
should require that CRYPTO be builtin (=y).

Currently, ZCACHE=y and CRYPTO=m is a valid configuration
when it should not be.

This patch changes the zcache Kconfig to enforce this
dependency.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>