meson: adjust nlohmann-json dependency

- Simplify dependency logic in meson.build.
- Use consistent dependency variable as style recommended by Meson.
- Use json wrap file matching the meson wrapdb.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8b6e14c03657a2017ab2c89ad98d5c460d3b7917
diff --git a/subprojects/nlohmann_json.wrap b/subprojects/nlohmann_json.wrap
new file mode 100644
index 0000000..3745380
--- /dev/null
+++ b/subprojects/nlohmann_json.wrap
@@ -0,0 +1,6 @@
+[wrap-git]
+revision = HEAD
+url = https://github.com/nlohmann/json.git
+
+[provide]
+nlohmann_json = nlohmann_json_dep