Suppress warning about MFD_CLOEXEC being redefined

glibc 2.27 #defined it as just "1U" instead of the kernel "0x0001U".
Same value, but because the tokens are different, this triggers a
warning.

Change-Id: I052407b777ec43f78378fffd153112449de8cf4f
Reviewed-by: Johan Helsing <johan.helsing@qt.io>

src/plugins/platforms/wayland/qwaylandshmbackingstore.cpp

@@ -59,7 +59,9 @@
 #ifdef Q_OS_LINUX
 #  include <sys/syscall.h>
 // from linux/memfd.h:
-#  define MFD_CLOEXEC       0x0001U
+#  ifndef MFD_CLOEXEC
+#    define MFD_CLOEXEC     0x0001U
+#  endif
 #endif
 
 QT_BEGIN_NAMESPACE