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

In commit 82930647 (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.
43 jobs for !6193 with boost_json_header in 16 minutes and 5 seconds (queued for 4 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #5849388
linux docker build cmake manual
build:centos6-x86_64

00:04:52

passed #5849385
linux docker build cmake
build:debian10-iwyu

00:08:01

passed #5849389
linux-3.17 linux docker build cmake manual
build:fedora33-ninja

00:01:48

passed #5849387
linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:04:05

passed #5849386
linux-3.17 linux docker build cmake
build:fedora33-tidy

00:16:05

manual #5849391
nonconcurrent xcode-12.4 shell macos cmake manual
build:macos-x86_64-makefiles
manual #5849390
nonconcurrent xcode-12.4 shell macos cmake manual
build:macos-x86_64-ninja
manual #5849392
nonconcurrent msvc-19.29-16.10 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja
 
  Test
passed #5849393
x11 linux-3.17 linux docker cmake
test:fedora33-ninja

00:05:23

created #5849395
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-makefiles
created #5849394
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-ninja
created #5849396
nonconcurrent msvc-19.29-16.10 vs2019 shell windows cmake
test:windows-vs2019-x64-ninja
 
  Test Ext
passed #5849399
linux docker cuda-rt cmake
test:cuda10.2-nvidia

00:03:21

passed #5849397
linux-3.17 linux docker build cmake
test:debian10-ninja

00:02:49

passed #5849398
linux-3.17 linux docker build cmake
test:fedora33-makefiles

00:04:08

passed #5849400
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi

00:02:23

created #5849401
concurrent xcode-12.4 shell macos cmake
test:macos-x86_64-xcode
created #5849402
concurrent msvc-19.29-16.10 vs2019 shell windows cmake
test:windows-vs2019-x64
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager