[HatoholArmPluginBase] Suppress a warning.

Without this patch, the following warning is shown.

HatoholArmPluginBase.cc: In member function 'mlpl::SmartTime HatoholArmPluginBase::getTimestampOfLastTrigger()':
HatoholArmPluginBase.cc:107:61: warning: narrowing conversion of 'body->HapiResTimestampOfLastTrigger::nanosec' from 'const uint32_t {aka const unsigned int}' to '__syscall_slong_t {aka long int}' inside { } is ill-formed in C++11 [-Wnarrowing]
  const timespec ts = {(time_t)body->timestamp, body->nanosec};
                                                             ^