net: lwm2m: fix printf warning

Cast a %lld argument to long long int. This is causing warnings on
recent GNU Arm Embedded toolchains, which fail the build with
-Werror=format=.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>

subsys/net/lib/lwm2m/lwm2m_rw_plain_text.c

@@ -154,7 +154,8 @@ size_t plain_text_put_float64fix(struct lwm2m_output_context *out,
 	char buf[sizeof("000000000")];
 
 	/* value of 123 -> "000000123" -- ignore sign */
-	len = snprintf(buf, sizeof(buf), "%09lld", abs(value->val2));
+	len = snprintf(buf, sizeof(buf), "%09lld",
+		       (long long int)abs(value->val2));
 	if (len != 9U) {
 		strcpy(buf, "0");
 	} else {