net: lwm2m: Fix ifdef nesting for data cache config

Avoid a "defined but not used" warning on
`init_next_pending_timeseries_data` when
LWM2M_RESOURCE_DATA_CACHE_SUPPORT is enabled but not LWM2M_VERSION_1_1.

Signed-off-by: Etienne de Maricourt <edmecomemail@gmail.com>