powerpc/pseries: plpks: export plpks_wrapping_is_supported

Building trusted-keys as a module fails modpost with:

ERROR: modpost: "plpks_wrapping_is_supported" [security/keys/trusted-keys/
trusted.ko] undefined!

Export plpks_wrapping_is_supported() so trusted-keys links cleanly

This patch is intended to be applied on top of the earlier "Extend "trusted
" keys to support a new trust source named the PowerVM Key Wrapping Module
(PKWM)" series (v5).
Link: https://lore.kernel.org/all/20260127145228.48320-1-ssrish@linux.ibm.com/

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202602010724.1g9hbLKv-lkp@intel.com/
Signed-off-by: Srish Srinivasan <ssrish@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260201165344.950870-1-ssrish@linux.ibm.com