mkelf32: fix compilation on 32 bit build host

mkelf32: fix compilation on 32 bit build host

When cross-compiling xen on a 32 bit build host:

boot/mkelf32.c: In function 'main':
boot/mkelf32.c:360:21: error: format '%ld' expects argument of type 'long int', but argument 3 has type 'Elf64_Off' [-Werror=format]
cc1: all warnings being treated as errors

Fix that by using PRId64 in format string.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>