RISC-V: Prepare dropping week attribute from arch_kexec_apply_relocations[_add]

Without this change arch/riscv/kernel/elf_kexec.c fails to compile once
commit 233c1e6c319c ("kexec_file: drop weak attribute from
arch_kexec_apply_relocations[_add]") is also contained in the tree.
This currently happens in next-20220527.

Prepare the RISC-V similar to the s390 adaption done in 233c1e6c319c.
This is safe to do on top of the riscv change even without the change to
arch_kexec_apply_relocations.

Fixes: 838b3e28 ("RISC-V: Load purgatory in kexec_file")
Looks-good-to: liaochang (A) <liaochang1@huawei.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/kexec.h

@@ -55,6 +55,13 @@ extern riscv_kexec_method riscv_kexec_norelocate;
 
 #ifdef CONFIG_KEXEC_FILE
 extern const struct kexec_file_ops elf_kexec_ops;
+
+struct purgatory_info;
+int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
+				     Elf_Shdr *section,
+				     const Elf_Shdr *relsec,
+				     const Elf_Shdr *symtab);
+#define arch_kexec_apply_relocations_add arch_kexec_apply_relocations_add
 #endif
 
 #endif