api.c: Remove static from cgroup_copy_controller_values()

api.c: Remove static from cgroup_copy_controller_values()

Remove static from cgroup_copy_controller_values() in api.c
so that other files within libcgroup can invoke it.  It will
be used by the abstraction layer code in a subsequent commit.

Also, declare *dst as a const pointer and *src as a const *
const pointer.  Subsequent abstraction layer checkins make
heavy use of const and without these changes, warnings (which
continuous integration converts to errors) are thrown.

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>