Fixed WinRT build after changing the header guard preprocessor symbol
diff --git a/src/audio/xaudio2/SDL_xaudio2.c b/src/audio/xaudio2/SDL_xaudio2.c
index 01ef9ef..6faa170 100644
--- a/src/audio/xaudio2/SDL_xaudio2.c
+++ b/src/audio/xaudio2/SDL_xaudio2.c
@@ -90,7 +90,7 @@
#endif
#endif
-#if !defined(_SDL_XAUDIO2_H)
+#if !defined(SDL_XAUDIO2_H_)
#define INITGUID 1
#include <xaudio2.h>
#endif