Remove include directory from clean list

Remove include directory from clean list

The current clean rule in the Unikraft build system can not remove
directories and terminates with an error if there is a directory in
the list of files to clean.

This commit removes the include directory inside the lib-musl build
directory from the clean list.

Since each sublibrary has a separate subdirectories in the include
directory, this change will not make headers accessible which would have
been deleted by the clean rule.

Alternatives considered:

1. Enable the build rule to remove directories.
This would lead to the uk_clean_list not containing a complete
lists of files to remove anymore and makes the potential damage
of errors in the clean list much higher.

2. Add every header file into the `LIBMUSL_CLEAN` variable.
This could be done in `_libmusl_import_lib` and would remove the
linked headers, but would still leave the directory tree.

3. Do not create the links, but include from the origin directory.
This would remove the ability to filter header files in the
sublibrary makefile.

Signed-off-by: Robert Kuban <robert.kuban@opensynergy.com>
Reviewed-by: Sergiu Moga <sergiu.moga@protonmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #17