meson: adjust nlohmann-json dependency

- Simplify the nlohmann-json dependency logic in meson.build
- Change wrap file name to align with other repositories.
- Use HEAD for nlohmann-json revision rather than a pinned version.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5ba6cc684c7c2dc8889e9a4cd882d5c6840b4b92
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