FindBoost: Change context header used

boost/context/all.hpp is removed in Boost 1.68.
boost/context/detail/fcontext.hpp is present in all Boost
releases containing context (≥1.61).

Modules/FindBoost.cmake

@@ -881,7 +881,7 @@ function(_Boost_COMPONENT_HEADERS component _hdrs)
   set(_Boost_CHRONO_HEADERS              "boost/chrono.hpp")
   set(_Boost_CONTAINER_HEADERS           "boost/container/container_fwd.hpp")
   set(_Boost_CONTRACT_HEADERS            "boost/contract.hpp")
-  set(_Boost_CONTEXT_HEADERS             "boost/context/all.hpp")
+  set(_Boost_CONTEXT_HEADERS             "boost/context/detail/fcontext.hpp")
   set(_Boost_COROUTINE_HEADERS           "boost/coroutine/all.hpp")
   set(_Boost_DATE_TIME_HEADERS           "boost/date_time/date.hpp")
   set(_Boost_EXCEPTION_HEADERS           "boost/exception/exception.hpp")