media: i2c: imx355: silence unused acpi_device_id warning

If driver is built without ACPI, the struct acpi_device_id won't be
used:

  drivers/media/i2c/imx355.c:1836:36: warning:
    'imx355_acpi_ids' defined but not used [-Wunused-const-variable=]

[Sakari Ailus: Added a space before __maybe_unused]

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>