BSE: processor.hh: move 'using Engine'
BSE: processor.hh: move 'using Engine'

G++-9.2 chokes on 'Foo f; using Foo = ...;' and demands the 'using'
declaration comes first - clang++-9 does not catch this.

Signed-off-by: Tim Janik <6ec6ff229c4e1c5bbd5933ee92e6c5832ff023bb@gnu.org>

tim-janik/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast,tim-janik/beast,tim-janik/beast,tim-janik/beast,tim-janik/beast,tim-janik/beast,tim-janik/beast,swesterfeld/beast