x86/mce: Allow for copy_mc_fragile symbol checksum to be generated

Add asm/mce.h to asm/asm-prototypes.h so that that asm symbol's checksum
can be generated in order to support CONFIG_MODVERSIONS with it and fix:

  WARNING: modpost: EXPORT symbol "copy_mc_fragile" [vmlinux] version \
	  generation failed, symbol will not be versioned.

For reference see:

  4efca4ed ("kbuild: modversions for EXPORT_SYMBOL() for asm")
  334bb773 ("x86/kbuild: enable modversions for symbols exported from asm")

Fixes: ec6347bb ("x86, powerpc: Rename memcpy_mcsafe() to copy_mc_to_{user, kernel}()")
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201007111447.GA23257@zn.tnic