RISCV: kexec: Fix build error without CONFIG_MODULES

RISCV: kexec: Fix build error without CONFIG_MODULES

When CONFIG_MODULES is not set/enabled:

../arch/riscv/kernel/elf_kexec.c:353:9: error: unknown type name 'Elf_Rela'; did you mean 'Elf64_Rela'?
  353 |         Elf_Rela *relas;
      |         ^~~~~~~~
      |         Elf64_Rela

Replace Elf_Rela by Elf64_Rela to avoid relying on CONFIG_MODULES.

Signed-off-by: Li Zhengyu <lizhengyu3@huawei.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220601063924.13037-1-lizhengyu3@huawei.com
Fixes: 408649770f38 ("RISC-V: Load purgatory in kexec_file")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>