mutex.h: Update for MSC 1938 compat

[johnwparent]

MSC 1938 removes native_handle from the API.
Remove reference in mutex.h

[BenKaufmann]

@johnwparent Thank you for making me aware of this issue 👍

This is truly a weird API break from MS, which judging from microsoft/STL#3770 (comment) and https://learn.microsoft.com/en-us/cpp/standard-library/mutex-class-stl?view=msvc-170#native_handle they also forgot to document 😞

However, given that according to C++ standard these members are indeed optional, I pushed a more general fix to clasp's dev branch, which is the branch were development happens.

[johnwparent]

@BenKaufmann Thanks for merging in a fix!

This is truly a weird API break from MS
Thoroughly perplexing choice on the part of MS, particularly considering how devoted to backwards compat they typically are. Thankfully it was such a break it was pretty easy to find.

Apologies for the patch, should have just gone with the generic "re-implementation", didn't realize that parts of Clasp depend on that.