Fix compiler problem in MSVC release mode

[hkaiser]

No description provided.

[sithhell]

Which linker error did you get without that include?

[hkaiser]

Actually, it wasn't a linker error, sorry. I got a compilation error saying the type base_lco_with_value is incomplete here:

hpx/src/runtime/trigger_lco.cpp

Lines 176 to 186 in 23c400e

	#if defined(HPX_MSVC) && !defined(HPX_DEBUG)
	///////////////////////////////////////////////////////////////////////////
	// Explicitly instantiate specific apply needed for set_lco_value for MSVC
	// (in release mode only, leads to missing symbols otherwise).
	template bool apply<
	lcos::base_lco_with_value<
	util::unused_type, util::unused_type
	>::set_value_action,
	util::unused_type
	>(naming::id_type const &, util::unused_type &&);
	#endif

[sithhell]

LGTM, thanks.