[SystemInfo] Fix the Desktop build.

Fix compilation errors in system_info on Desktop build.

Errors:
../../system_info/system_info_battery_desktop.cc:22:33: error: definition of implicitly-declared ‘SysInfoBattery::~SysInfoBattery()’
../../system_info/system_info_battery_desktop.cc: In static member function ‘static gboolean SysInfoBattery::OnUpdateTimeout(gpointer)’:
../../system_info/system_info_battery_desktop.cc:124:34: error: cannot call member function ‘void SysInfoObject::PostMessageToListeners(const picojson::value&)’ without object