um: Restore symbol versions for __memcpy and memcpy

If CONFIG_MODVERSIONS=y:

    WARNING: EXPORT symbol "__memcpy" [vmlinux] version generation failed, symbol will not be versioned.
    WARNING: EXPORT symbol "memcpy" [vmlinux] version generation failed, symbol will not be versioned.

Add <asm/asm-prototypes.h>, including the generic version, so that
genksyms knows the types of these symbols and can generate CRCs for
them.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Richard Weinberger <richard@nod.at>