package/xen: fix build with gcc7

Fixes:

  http://autobuild.buildroot.net/results/f9553c1669bd05acee3ccea76e36c2474e534d8c/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: improve patch, reformatted with Git and better commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>