Change the scope of ModuleCompiler->functionName to protected.

This is related to
https://github.com/schmittjoh/JMSTwigJsBundle/issues/25. To be able to
output AMD compatible templates, I extended the `ModuleCompiler` (see
https://github.com/kimlai/melikeyradio/blob/master/src/MeLikey/WebAppBundle/TwigJs/Compiler/AmdModuleCompiler.php),
but to do so I need the access the `functionName` property in the child
class.