Update src/Generator/TypeBuilder.php

[koekaverna]

Change self:: to static:: when refreing to class constant

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Documented?	no
Fixed tickets	#...
License	MIT

In my project I need to extends Overblog\GraphQLBundle\Generator\TypeBuilder to overide protected const EXTENDS;
I use custom Enum in my models and I need to overide EnumType serrializer to make it works. So I updated TypeBuilder to use static::EXTENDS instead self::EXTENDS

[koekaverna]

@murtukov thx!