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/\
3a4e128/lib/libc/common/\
source/time/time.c#L17

Also mentioned in zephyrproject-rtos#89068

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

Author: cfriedt

samples/net/lwm2m_client/overlay-data-cache.conf

@@ -1,5 +1,5 @@
 CONFIG_ZCBOR=y
 CONFIG_ZCBOR_CANONICAL=y
 CONFIG_LWM2M_RW_SENML_CBOR_SUPPORT=y
-CONFIG_POSIX_TIMERS=y
+CONFIG_XSI_SINGLE_PROCESS=y
 CONFIG_LWM2M_RESOURCE_DATA_CACHE_SUPPORT=y

subsys/net/lib/lwm2m/Kconfig

@@ -227,7 +227,7 @@ config LWM2M_RD_CLIENT_MAX_RETRIES
 config LWM2M_RESOURCE_DATA_CACHE_SUPPORT
 	bool "Resource Time series data cache support"
 	depends on (LWM2M_RW_SENML_JSON_SUPPORT || LWM2M_RW_SENML_CBOR_SUPPORT)
-	depends on (POSIX_TIMERS && RING_BUFFER)
+	depends on (XSI_SINGLE_PROCESS && RING_BUFFER)
 	help
 	  Enable time series data storage.
 	  Requires time() to provide current Unix timestamp.