arm64/efi: Make strnlen() available to the EFI namespace

Changes introduced in the upstream version of libfdt pulled in by commit
91feabc2e2 ("scripts/dtc: Update to upstream commit b06e55c88b9b") use
the strnlen() function, which isn't currently available to the EFI name-
space. Add it to the EFI namespace to avoid a linker error.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Rob Herring <robh@kernel.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/image.h

@@ -89,6 +89,7 @@ __efistub_memcpy		= KALLSYMS_HIDE(__pi_memcpy);
 __efistub_memmove		= KALLSYMS_HIDE(__pi_memmove);
 __efistub_memset		= KALLSYMS_HIDE(__pi_memset);
 __efistub_strlen		= KALLSYMS_HIDE(__pi_strlen);
+__efistub_strnlen		= KALLSYMS_HIDE(__pi_strnlen);
 __efistub_strcmp		= KALLSYMS_HIDE(__pi_strcmp);
 __efistub_strncmp		= KALLSYMS_HIDE(__pi_strncmp);
 __efistub___flush_dcache_area	= KALLSYMS_HIDE(__pi___flush_dcache_area);