CppTools: Avoid self-include

...in CPlusPlus::Document due to cyclic includes.

Task-number: QTCREATORBUG-11457
Change-Id: I1ca19c901c26d9984d795a61879dd6b41c57096c
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>