arm64: drop non-existing vdso-offsets.h from .gitignore

Since commit a66649da ("arm64: fix vdso-offsets.h dependency"),
include/generated/vdso-offsets.h is directly generated without
arch/arm64/kernel/vdso/vdso-offsets.h.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>