x86/mce: Add copy_mc_fragile_handle_tail() prototype

copy_mc_fragile_handle_tail() is only called from assembler,
but 'make W=1' complains about a missing prototype:

arch/x86/lib/copy_mc.c:26:1: warning: no previous prototype for ‘copy_mc_fragile_handle_tail’ [-Wmissing-prototypes]
   26 | copy_mc_fragile_handle_tail(char *to, char *from, unsigned len)

Add the prototype to avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://lore.kernel.org/all/20230516193549.544673-16-arnd%40kernel.org