Fix use of QMutex on Linux before FUTEX_PRIVATE_FLAG was added

It was added in 2.6.22. If we pass it to 2.6.21, we'll get -ENOSYS,
which is bad for QMutex. This fix simply defines it to 0 if the header
doesn't define it.

But as a consequence: if Qt is built with newer kernel headers, it won't
run on older versions. It's not likely that someone is still using Qt
5.7 on a 2.6.21 kernel (v2.6.21.7 was released on 2007-08-04).

Change-Id: Icb178bb113bb437c9b67fffd1451dd7bb964f0c8
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/thread/qmutex_linux.cpp

@@ -57,7 +57,7 @@
 #endif
 
 #ifndef FUTEX_PRIVATE_FLAG
-#  define FUTEX_PRIVATE_FLAG    128
+#  define FUTEX_PRIVATE_FLAG    0
 #endif