Enable `EXT_MODULES` only if `BUILD_GOLDFISH_DRIVERS` is requested

The `virtual_device` targets are used for both
Cuttlefish and Goldfish. Cuttlefish alone does not
produce any `EXT_MODULES` and therefore on
Cuttlefish-only targets (e.g. `kernel_virt_kprobes_*`)
`EXT_MODULES` causes a build error.

Bug: 204762438
Test: presubmit
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: I5eb0b2aff74ef8bc9d41091e6df9b1b599d44ede