crash: align the declaration of crash_load_dm_crypt_keys with CONFIG_CRASH_DM_CRYPT

crash: align the declaration of crash_load_dm_crypt_keys with CONFIG_CRASH_DM_CRYPT

This will prevent a compilation failure when CONFIG_CRASH_DUMP is enabled
but CONFIG_CRASH_DM_CRYPT is disabled,

       arch/powerpc/kexec/elf_64.c: In function 'elf64_load':
    >> arch/powerpc/kexec/elf_64.c:82:23: error: implicit declaration of function 'crash_load_dm_crypt_keys' [-Werror=implicit-function-declaration]
          82 |                 ret = crash_load_dm_crypt_keys(image);
             |                       ^~~~~~~~~~~~~~~~~~~~~~~~
       cc1: some warnings being treated as errors

Link: https://lkml.kernel.org/r/20260225060347.718905-3-coxu@redhat.com
Signed-off-by: Coiby Xu <coxu@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202602120648.RgQALnnI-lkp@intel.com/
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Arnaud Lefebvre <arnaud.lefebvre@clever-cloud.com>
Cc: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Cc: Dave Young <dyoung@redhat.com>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Pingfan Liu <kernelfans@gmail.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
Cc: Thomas Staudt <tstaudt@de.ibm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>