samples: net: lwm2m: depend on xsi single process for gettimeofday()

https://github.com/zephyrproject-rtos/zephyr/actions/runs/\
15014534061/job/42189411225

We were seeing a linker error of the form

```
picolibc/x86_64-zephyr-elf/lib/32/libc.a(libc_time_time.c.o): \
  in function `time':
time.c:(.text.time+0x11): undefined reference to `gettimeofday'
```

Update the dependency from CONFIG_POSIX_TIMERS to
CONFIG_XSI_SINGLE_PROCESS for gettimeofday().

Note: this is really only a workaround. The proper solution would
be to have libc functions not depend on POSIX functions.

Specifically, here

https://github.com/zephyrproject-rtos/picolibc/blob/\
51a8b32857e75345c37652a80b5cda98b28d69e5/newlib/libc/time/\
time.c#L54

and here

https://github.com/zephyrproject-rtos/zephyr/blob/\


3a4e1289/lib/libc/common/\
source/time/time.c#L17

Also mentioned in #89068

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>