meson: adjust nlohmann-json dependency

- Simplify the dependency logic in the meson.build.
- Align the meson dep name with consistent practices from other repos.
- Use the non-versioned wrap file.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I0e3052763bffe9f98d8ca10c5b523b58e000f636
diff --git a/meson.build b/meson.build
index e547e4f..ef39248 100644
--- a/meson.build
+++ b/meson.build
@@ -46,12 +46,7 @@
     cereal_dep = cereal_proj.dependency('cereal')
 endif
 
-if cpp.has_header('nlohmann/json.hpp')
-    json_dep = declare_dependency()
-else
-    json_dep = dependency('nlohmann_json')
-endif
-
+nlohmann_json_dep = dependency('nlohmann_json', include_type: 'system')
 phosphor_dbus_interfaces_dep = dependency('phosphor-dbus-interfaces')
 phosphor_logging_dep = dependency('phosphor-logging')
 sdbusplus_dep = dependency('sdbusplus')