mythread.h: Fix typo error in Vista threads mythread_once().

The "once_" variable was accidentally referred to as just "once". This
prevented building with Vista threads when
HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR was not defined.

src/common/mythread.h

@@ -378,7 +378,7 @@ typedef struct {
 			abort(); \
 		if (pending_) { \
 			func(); \
-			if (!InitOnceComplete(&once, 0, NULL)) \
+			if (!InitOnceComplete(&once_, 0, NULL)) \
 				abort(); \
 		} \
 	} while (0)