arm64: Select MODULES_USE_ELF_RELA

With commit 786d35d4 (make most arch asm/module.h files use
asm-generic/module.h) arm64 needs to enable MODULES_USE_ELF_RELA for
loadable modules.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Will Deacon <Will.Deacon@arm.com>