nvdimm: make security_show static

nvdimm: make security_show static

The security_show function is not used outside of drivers/nvdimm/dimm_devs.c
and the attribute it is for is also already static. Silence the sparse
warning for this not being declared by making it static. Fixes:

  drivers/nvdimm/dimm_devs.c:352:9: warning: symbol 'security_show' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lore.kernel.org/r/20230616160925.17687-1-ben.dooks@codethink.co.uk
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>