selinux: simplify cred_init_security

The parameter of selinux_cred() is declared const, so an explicit cast
dropping the const qualifier is not necessary. Without the cast the
local variable cred serves no purpose.

Reported by clang [-Wcast-qual]

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>