libcasper: explicit cast in cap_fileargs header

The implicit cast is incompatible with standard compiler options in a
C++ project, making the library difficult to use.

Signed-off-by: Quentin Thébault <quentin.thebault@defenso.fr>
Sponsored by: Defenso
MFC after: 3 days
Discussed with: kevans
Reviewed by: emaste, vexeduxr
Pull request: https://github.com/freebsd/freebsd-src/pull/1857