Include musl's own headers when building

Include musl's own headers when building

This fixes a build error where an internal use of `prctl` fails due to a
missing prototype declaration.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Rares Miculescu <miculescur@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #45