KVM: arm64: nvhe: Fix missing FORCE for hyp-reloc.S build rule

Add FORCE so that if_changed can detect the command line change.

We'll otherwise see a compilation warning since commit e1f86d7b


("kbuild: warn if FORCE is missing for if_changed(_dep,_rule) and
filechk").

arch/arm64/kvm/hyp/nvhe/Makefile:58: FORCE prerequisite is missing

Cc: David Brazdil <dbrazdil@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210907052137.1059-1-yuzenghui@huawei.com