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.com


Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>