The prototype of mem_map_via_hcall() is missing in its header, so add
it.
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Fixes: a43fb7da53007e67ad ("xen/pvh: Move Xen code for getting mem map via hcall out of common file")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>