Install `*.stub.php` for use with the `phpize` build environment

[TimWolla]

Description

I'm trying to use gen_stub.php to define an attribute class in an extension. gen_stub.php requires the zend_attributes.stub.php to be available to resolve the \Attribute::TARGET_X constants. For that purpose gen_stub.php has support for including other files as a dependency, which is already used for zend_test:

php-src/ext/zend_test/test.stub.php

Line 9 in e3715cd

require "Zend/zend_attributes.stub.php";

However this is not usable for external extensions, since the stub files are not part of a PHP installation / the phpize build environment. Only gen_stub.php itself and the generated arginfo.h headers are.