Revert "fix for mingw-w64"

This reverts commit 32b13ccb46.

Reported by Wolf0 (aka wolf9466) to not break MinGW 64-bit compilation
with GCC 4.8.2 (latest stable).

sgminer.c

@@ -3138,7 +3138,7 @@ static void kill_mining(void)
 		if (thr && PTH(thr) != 0L)
 			pth = &thr->pth;
 		thr_info_cancel(thr);
-#if !defined(WIN32) || defined(__MINGW64_VERSION_MAJOR)
+#ifndef WIN32
 		if (pth && *pth)
 			pthread_join(*pth, NULL);
 #else