In https://github.com/mongodb/mongo-cxx-driver/commit/45c3f04fd829e65aa70fca99b44f05ba5ed5ea56 I rather foolishly added a #pragma once to the initializer_functions.h header while adding license text.

This broke the static initializers on Windows since this file is intended to be included multiple times to tie in the initializers.

The fix is to remove the #pragma once.