community/nlohmann-json: install multiple split headers instead of a single large one

At least the next OpenRCT2 version will require this
7 jobs for !13399 with community_nlohmann-json in 3 minutes and 16 seconds
detached
Status Job ID Name Coverage
  Lint
passed #224183
docker-alpine x86_64
lint

00:00:24

 
  Build
passed #224188
docker-alpine ci-build aarch64
build-aarch64

00:02:42

passed #224189
docker-alpine ci-build armv7
build-armv7

00:02:40

passed #224187
docker-alpine ci-build ppc64le
build-ppc64le

00:00:39

passed #224186
docker-alpine ci-build s390x
build-s390x

00:01:59

passed #224185
docker-alpine ci-build x86
build-x86

00:02:49

passed #224184
docker-alpine ci-build x86_64
build-x86_64

00:01:14