net: lwm2m: Fix build issue on time-series cache

This was only missing a variable declaration.

Fixes #55031

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

subsys/net/lib/lwm2m/lwm2m_registry.c

@@ -2136,6 +2136,7 @@ int lwm2m_engine_enable_cache(const char *resource_path, struct lwm2m_time_serie
 {
 #if defined(CONFIG_LWM2M_RESOURCE_DATA_CACHE_SUPPORT)
 	struct lwm2m_obj_path path;
+	int ret;
 
 	/* translate path -> path_obj */
 	ret = lwm2m_string_to_path(resource_path, &path, '/');