boost: Fix compilation of Boost.Variants move assignment

boost: Fix compilation of Boost.Variants move assignment

Fix compilation of Boost.Variants move assignment for situations when one of the variant template classes has nothrow copy constructor and throwing move constructor (refs #8772)

Fixes compilation error:

.../output/host/usr/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/variant/variant.hpp: In member function 'void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::move_assigner::internal_visit(RhsT&, int) [with RhsT = boost::shared_ptr<void>, T0_ = boost::shared_ptr<void>, T1 = boost::signals2::detail::foreign_void_shared_ptr, T2 = boost::detail::variant::void_, ..., T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_]':
...
.../output/host/usr/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/variant/variant.hpp:2058:13: error: no matching function for call to 'boost::variant<boost::shared_ptr<void>, boost::signals2::detail::foreign_void_shared_ptr>::move_assigner::assign_impl(boost::shared_ptr<void>&, nothrow_copy, nothrow_move_constructor, boost::variant<boost::shared_ptr<void>, boost::signals2::detail::foreign_void_shared_ptr>::has_fallback_type_)'

Reported here: https://svn.boost.org/trac/boost/ticket/8772

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Backported-from: https://svn.boost.org/trac/boost/changeset/85080
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>