Merge SDL_wasapi_win32 into SDL_wasapi

[Lzard]

Moves code from SDL_wasapi_win32.c into SDL_wasapi.c, following support removal of WinRT.

SDL/src/audio/wasapi/SDL_wasapi_win32.c

Lines 22 to 29 in 8a2cac7

	/* !!! FIXME: merge this all into SDL_wasapi.c, now that WinRT is gone.
	This is code that Windows uses to talk to WASAPI-related system APIs.
	This is for non-WinRT desktop apps. The C++/CX implementation of these
	functions, exclusive to WinRT, are in SDL_wasapi_winrt.cpp.
	The code in SDL_wasapi.c is used by both standard Windows and WinRT builds
	to deal with audio and calls into these functions. */

[icculus]

This is something I wanted to do but didn't have time to clean up, so I'm 100% okay with this PR.

Let's definitely decide we aren't reverting #10724 before we merge this, but otherwise I approve.