benchmark

Fixed build issues on window (#1449)

* Fixed build issues on window

- Added missing dlimport/export attributes in function definitions. (They are needed in both decls and defs)
- Removed dlimport/dlexprt attribute in private field. (global_context is not exported anywhere).

* fixed incorrect include path

* undo changes w.r.t HelperPrintf

* removed forward decl of private variable - instead, introduce a getter and use it.

* Removed forward decl from benchmark_gtest too

Co-authored-by: Dominic Hamon <dominichamon@users.noreply.github.com>