util/grub-module-verifierXX: Changed get_shnum() return type

util/grub-module-verifierXX: Changed get_shnum() return type

In util/grub-module-verifierXX.c, the function get_shnum() returns the variable
shnum, which is of the type Elf_Word. In the function, shnum can be obtained by
the e_shnum member of an Elf_Ehdr or the sh_size member of an Elf_Shdr. The
sh_size member can either be grub_uint32_t or grub_uint64_t, depending on the
architecture, but Elf_Word is only grub_uint32_t. To account for when sh_size is
grub_uint64_t, we can set shnum to have type Elf_Shnum and have get_shnum()
return an Elf_Shnum.

Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>