Enable nlohmann-json dependency in phosphor-networkd

- This commit would enable and enforce the nlohmann-json headers
  as a dependency.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: I73e784d92e3480faa7eb25c127df0c1cbebeb1c7
1 file changed