um: Add include: memset() and memcpy() are in <string.h>

These two functions are otherwise unknown to the pedantic compiler.
Include the correct header to enable the build to succeed.

Signed-off-by: Zach van Rijn <me@zv.io>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>