arm64: Make ARM64_ERRATUM_1188873 depend on COMPAT

Since ARM64_ERRATUM_1188873 only affects AArch32 EL0, it makes some
sense that it should depend on COMPAT.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>