security_selinux: Declare internal autoptr cleanup helper for 'context_t'

security_selinux: Declare internal autoptr cleanup helper for 'context_t'

The selinux headers have a 'typedef context_s_t *context_t;' definition
in the header so we declare autoptr cleanup function for 'context_s_t'
and use it instead of 'context_t' definitions.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>