xen/pvh: add missing prototype to header

The prototype of mem_map_via_hcall() is missing in its header, so add
it.
Reported-by: kernel test robot <lkp@intel.com>
Fixes: a43fb7da ("xen/pvh: Move Xen code for getting mem map via hcall out of common file")
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20211119153913.21678-1-jgross@suse.comReviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

arch/x86/include/asm/xen/hypervisor.h

@@ -64,6 +64,7 @@ void xen_arch_unregister_cpu(int num);
 
 #ifdef CONFIG_PVH
 void __init xen_pvh_init(struct boot_params *boot_params);
+void __init mem_map_via_hcall(struct boot_params *boot_params_p);
 #endif
 
 #endif /* _ASM_X86_XEN_HYPERVISOR_H */