xen: Fix "error: unable to find tring literal operator"

Total time: 30.77 secs
make -r -C build/release/ all
make[1]: Entering directory `/home/zwu/osv/build/release'
  CXX bsd/sys/xen/gnttab.o
In file included from /home/zwu/osv/bsd/sys/xen/hypervisor.h:40:0,
                 from /home/zwu/osv/bsd/sys/xen/gnttab.cc:29:
/home/zwu/osv/bsd/machine/xen/hypercall.h: In function ‘int HYPERVISOR_set_trap_table(const trap_info_t*)’:
/home/zwu/osv/bsd/machine/xen/hypercall.h:146:9: error: unable to find string literal operator ‘operator"" STR’
/home/zwu/osv/bsd/machine/xen/hypercall.h: In function ‘int HYPERVISOR_mmu_update(mmu_update_t*, unsigned int, unsigned int*, domid_t)’:
/home/zwu/osv/bsd/machine/xen/hypercall.h:154:9: error: unable to find string literal operator ‘operator"" STR’
/home/zwu/osv/bsd/machine/xen/hypercall.h: In function ‘int HYPERVISOR_mmuext_op(mmuext_op*, unsigned int, unsigned int*, domid_t)’:
......
make[1]: *** [bsd/sys/xen/gnttab.o] Error 1
make[1]: Leaving directory `/home/zwu/osv/build/release'
make: *** [all] Error 2

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Zhi Yong Wu <zwu.kernel@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>