Update win32_api.hpp

This modification is proposed here:
http://www.boost.org/doc/libs/develop/boost/interprocess/detail/win32_api.hpp

Description here:
https://svn.boost.org/trac/boost/ticket/10325

SC-related discussion here:
https://github.com/supercollider/supercollider/pull/1165#issuecomment-54707582

Victor Bombi found this, he can build Server and Supernova after the change and I can confirm this.