Fix RtMidi flags for Windows

Dear Martin,

Here is a small fix for Windows in the project file PhMidi.

For your information, I have successfully tested on Windows the connection between MidiTool and Joker, and I will try to test the same from a DAW now. (it seems that there is a memory leak somewhere in Joker, it ended taking more than 1 GB of memory after a couple of minutes of playing...)

Best regards,

Timothée