fix(libc): add the memcpy declaration back

Patch 34d7f196b moved definitions to a new file but missed that of
memcpy. Put it back.

Change-Id: I5e9797e611e6f1f97232f03de98f81db86a66c2a
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
1 file changed