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.

(cherry picked from commit c0c0cd4a483a672b66a13761583bc4f84d86d501)

src/common/mythread.h

@@ -372,7 +372,7 @@ typedef struct {
 			abort(); \
 		if (pending_) { \
 			func(); \
-			if (!InitOnceComplete(&once, 0, NULL)) \
+			if (!InitOnceComplete(&once_, 0, NULL)) \
 				abort(); \
 		} \
 	} while (0)