Consider not making functions in headers static to improve compatibility with C++ modules

[jcelerier]

If a function is marked static, the header put into the global fragment and a template ends up causing instantiation and thus call of this static function, well, with modules this does not work.

Here's a patch : https://github.com/cameron314/concurrentqueue/commit/324c8187e294a78618c99d19dd7cd43809784333

[cameron314]

Seems reasonable. Please open a PR with that diff.