fix build in register_event_callback after d5b334cebb96840885f6a5cd7557dd4672bfd931

without this, the build will first fail with

  In file included from /home/evgeni/Devel/sysdig/userspace/libsinsp/sinsp.h:84:0,
                   from /home/evgeni/Devel/sysdig/userspace/libsinsp/chisel.cpp:31:
  /home/evgeni/Devel/sysdig/userspace/libsinsp/fdinfo.h:160:47: error: ‘sinsp_fdinfo_t<int>::callback_type’ has not been declared
    void register_event_callback(sinsp_fdinfo_t::callback_type etype, sinsp_protodecoder* dec);
                                               ^
  userspace/libsinsp/CMakeFiles/sinsp.dir/build.make:54: recipe for target   'userspace/libsinsp/CMakeFiles/sinsp.dir/chisel.cpp.o' failed
   make[2]: *** [userspace/libsinsp/CMakeFiles/sinsp.dir/chisel.cpp.o] Error 1

and later with

/home/evgeni/Devel/sysdig/userspace/libsinsp/fdinfo.cpp:196:6: error: specializing member ‘sinsp_fdinfo<int>::register_event_callback’ requires ‘template<>’ syntax
 void sinsp_fdinfo_t::register_event_callback(sinsp_fdinfo_t::callback_type etype, sinsp_protodecoder* dec)
      ^
userspace/libsinsp/CMakeFiles/sinsp.dir/build.make:169: recipe for target 'userspace/libsinsp/CMakeFiles/sinsp.dir/fdinfo.cpp.o' failed
make[2]: *** [userspace/libsinsp/CMakeFiles/sinsp.dir/fdinfo.cpp.o] Error 1