Adding type hints to @Timed ast transformation.

Without the type hints, @Timed cannot be used in Grails 2.3 apps with
services that has the new grails @Transactional annotation (i.e. not the
Spring @Transactional annotation). This is because @Transactional
changes the AST to have @CompileStatic, which in turn breaks with the
dynamic declarations this commit fixes.