Use windows vsnprintf fixup logic on all windows environments

Previously I'd relied on mingw to provide a vsnprintf with a
conformant return value.  But it appears that some mingw
environments don't do that.

evutil.c

@@ -1562,7 +1562,7 @@ evutil_vsnprintf(char *buf, size_t buflen, const char *format, va_list ap)
 	int r;
 	if (!buflen)
 		return 0;
-#ifdef _MSC_VER
+#if defined(_MSC_VER) || defined(WIN32)
 	r = _vsnprintf(buf, buflen, format, ap);
 	if (r < 0)
 		r = _vscprintf(format, ap);