minimp3: Add a .cpp file to simplify building the single-header implementation

Single-header libraries like this require passing a bunch of defines _once_
before including the header, but not multiple times. This can make it tricky
in user code to know when to request the implementation, if the header needs
to be included in multiple files.

So properly compiling a .cpp file for the implementation solves this, and
also enables us to properly disable warnings on thirdparty implementation code.

thirdparty/README.md

@@ -416,6 +416,7 @@ Files extracted from upstream repository:
 
 Some changes have been made in order to fix Windows on ARM build errors, and
 to solve some MSVC warnings. See the patches in the `patches` directory.
+`minimp3_ex.cpp` was created to simplify the use of the single header library.
 
 
 ## miniupnpc