Initially, those strings describing different parts of an MCE message
were shared with amd64_edac and were therefore exported to modules.
However, all except pp_msgs are used only in one place right now so hide
them and make them static.

No functionality change.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Borislav Petkov <bp@alien8.de>