Remove inclusion of uk/user.h

Remove inclusion of uk/user.h

Currently, we include uk/user.h from the core's posix-user library to
declare signatures for get/setresuid and get/setresgid. However,
these definitions should be part of the C library and not Unikraft.
This commit removes the dependency on the uk/user.h header
and moves the function declaration into the unistd.h directly. It also
adds the missing _GNU_SOURCE guard.

Signed-off-by: Marc Rittinghaus <marc.rittinghaus@kit.edu>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Approved-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #22