credmgr: Fix copy and paste error in add_validator

This won't hurt as long as sets and validators are of the same class.
But as soon as one of the object's class is changed this will cause
either a compile error (best option), or result (most likely) in a
crash.

Signed-off-by: Thomas Egerer <thomas.egerer@secunet.com>

src/libstrongswan/credentials/credential_manager.c

@@ -1279,7 +1279,7 @@ METHOD(credential_manager_t, add_validator, void,
 	private_credential_manager_t *this, cert_validator_t *vdtr)
 {
 	this->lock->write_lock(this->lock);
-	this->sets->insert_last(this->validators, vdtr);
+	this->validators->insert_last(this->validators, vdtr);
 	this->lock->unlock(this->lock);
 }