lib/ukalloc: Use Unikraft internal types

lib/ukalloc: Use Unikraft internal types

Use Unikraft-internal types for the API headers in order to have minimal
dependency to libc definitions. This is done so that libcs can integrate
`<uk/alloc.h> directly with their header files without breaking the
declaration set of libc headers.
This is done towards receiving per-library statistics
(see the following commit).

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #229