i2c_shell: only include i3c if I3C=y

Only lookup for i3c devices if I3C=y, fixes a build error:

zephyr/drivers/i2c/i2c_shell.c:
In function 'device_is_i2c':
zephyr/drivers/i2c/i2c_shell.c:341:43:
error: array type has incomplete element type 'struct i3c_driver_api'
341 |         return DEVICE_API_IS(i2c, dev) || DEVICE_API_IS(i3c, dev);

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>