Make the allocators used by malloc and linux_umalloc valgrindable.
Include valgrind headers in the codebase to avoid a build dependency
on valgrind.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>