KVM: selftests: Add strnlen() to the string overrides

Add strnlen() to the string overrides to allow it to be called in the
guest.

The implementation for strnlen() was taken from the kernel's generic
version, lib/string.c.

This will be needed when printf() is introduced.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Link: https://lore.kernel.org/r/20230729003643.1053367-5-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>