comms/telldus-core: Fix build with Clang 6

common/EventHandler_unix.cpp:83:24: error: non-constant-expression cannot be narrowed from type 'unsigned long long' to 'time_t' (aka 'int') in initializer list [-Wc++11-narrowing]

timespec abstime = { now.tv_sec + (abstime_ns_large / 1000000000), abstime_ns_large % 1000000000 };
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/telldus-core-2.1.2_5.log

PR: 230882
Submitted by: tobik
Approved by: Johan Ström <johan@stromnet.se> (maintainer)