C++, 51 bytes

#include<os>
void Service::start(){OS::shutdown();}

Only works in IncludeOS (v0.10.0-rc.1)

Test:

$ pwd
/home/simon/IncludeOS/examples/demo_service

$ cat service.cpp
#include<os>
void Service::start(){OS::shutdown();}

$ mkdir build && cd build && cmake .. && make && cd ..
[...]

$ boot build/IncludeOS_example
[...]
================================================================================
================================================================================
 IncludeOS v0.10.0-rc.1-dirty
 +--> Running [ IncludeOS minimal example ]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

$