cxl: Fix compile warning for cxl_security_ops extern

cxl: Fix compile warning for cxl_security_ops extern

Jonathan reported he has observed compiler warning when using running with
W=1 C=1 for cxl_security_ops that is declared as an extern in cxl/pmem.c.
Move to cxl.h to make it visible to all cxl sources.

Suggested-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Tested-by: Alison Schofield <alison.schofield@intel.com>
Link: https://lore.kernel.org/linux-cxl/167771196186.3285982.18283746206612049722.stgit@djiang5-mobl3.local/
Signed-off-by: Dave Jiang <dave.jiang@intel.com>