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 Name Job ID Coverage
  Build
passed build:centos6-x86_64 #5849388
cmake build docker linux manual

00:04:52

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

00:08:01

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

00:01:48

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

00:04:05

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

00:16:05

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

00:05:23

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

00:03:21

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

00:02:49

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

00:04:08

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

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