package/xen: fix qemu-xen memfd build failure

Fix the QEMU memfd compile error:
tools/qemu-xen/util/memfd.c:40:12: error: static declaration of 'memfd_create' follows non-static declaration
 static int memfd_create(const char *name, unsigned int flags)
            ^~~~~~~~~~~~

that has been introduced since the Glibc 2.27 upgrade.

This just involves porting the upstream QEMU patch to the Xen QEMU tree.

This fixes:
http://autobuild.buildroot.net/results/ec7cda00e07b0c98a9a366244b67611e042e0d4b/



Signed-off-by: Alistair Francis <alistair@alistair23.me>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>