Mostly based on bugfix from Bean.
* kern/elf.c (grub_elf32_phdr_iterate): Use `NESTED_FUNC_ATTR'
attribute with hook() parameter.
(grub_elf32_load): Use `NESTED_FUNC_ATTR' with grub_elf32_load_segment()
declaration.
(grub_elf64_phdr_iterate): Use `NESTED_FUNC_ATTR'
attribute with hook() parameter.
(grub_elf64_load): Use `NESTED_FUNC_ATTR' with grub_elf64_load_segment()
declaration.