Imply ALLOW_MISSING_DEPENDENCIES for Mac builds

At least until the device builds are largely disabled and host builds
become opt-in rather than opt-out. Otherwise every host test that
depends on a linux-only library breaks the mac build, even if it won't
be used.

Bug: 187222815
Change-Id: Ic598f2b66a949b90e5b3fbebf463f28278e4834f