package/json-for-modern-cpp: enable multiple headers by default

package/json-for-modern-cpp: enable multiple headers by default

json-for-modern-cpp provides an option to enable multiple headers support.
Currently this option is disabled and the package creates just a
single header: "nlohmann/json.hpp", which contains everything. Enabling
this option, splits this single header into multiple ones. First of all
this provides an additional header: "nlohmann/json_fwd.hpp", which contains
only forwarding declarations of types provided by this library. This
gives you a more finer control over include dependencies and can speed up
build times significantly. The top level header: "nlohmann/json.hpp" is
still there and therefore code which used it before, would stay compatible.

Signed-off-by: Dimitrij Kotrev <dimitrij.kotrev@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>