makesyscalls: use @generated tag in generated files

Multiple tools use @generated to identify generated files (for example,
in a review Phabricator will by default hide diffs in generated files).
Use the @generated tag in makesyscalls.sh as we've done for other
generated files.

Reviewed by: cem
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20183