FindBoost: Add check for json component header in Boost 1.75+

In commit 8293064760 (FindBoost: Add support for Boost 1.75, 2021-02-03,
v3.19.5~9^2) the dependencies of the json component were added, but the
header listing was left out.

Modules/FindBoost.cmake

@@ -1405,6 +1405,7 @@ function(_Boost_COMPONENT_HEADERS component _hdrs)
   set(_Boost_LOCALE_HEADERS              "boost/locale.hpp")
   set(_Boost_LOG_HEADERS                 "boost/log/core.hpp")
   set(_Boost_LOG_SETUP_HEADERS           "boost/log/detail/setup_config.hpp")
+  set(_Boost_JSON_HEADERS                "boost/json.hpp")
   set(_Boost_MATH_HEADERS                "boost/math_fwd.hpp")
   set(_Boost_MATH_C99_HEADERS            "boost/math/tr1.hpp")
   set(_Boost_MATH_C99F_HEADERS           "boost/math/tr1.hpp")