Avoid order of initialization problem in CINT/Core/Meta.

Avoid to forward declare TTree in an header file in core/base as this tickles
(on Ubuntu) an order of initialization problems [no point in solving the
rarely appearing underlying issue in v5]

Import in v6 to keep code in sync.

(cherry picked from commit acd8ecba7025d576050186035ffdad82a034aa7c)