ipm: Fixed linker error in drivers/ipm/ipm_handlers.c

```
/home/humanentity/zephyr/drivers/ipm/ipm_handlers.c:23: undefined reference to 'z_impl_max_data_size_get'
```

The error shows when having CONFIG_BT=y and CONFIG_USERSPACE=y, and
is easily reproduced by adding
```C
CONFIG_USERSPACE=y
```
to `samples/bluetooth/peripheral/prj.conf` and building the sample.
E.g.
```
$ west build -p auto -b nrf5340pdk_nrf5340_cpuapp samples/bluetooth/peripheral
```

Signed-off-by: Peter Andersson <pelleplutt1976@gmail.com>