build: simplify nlohmann-json subproject and update

The bitbake package for nlohmann-json installs a nlohmann_json.pc
file and the subproject has a meson variable.  We can simplify the
dependency in the meson file significantly by using the correct
dependency name and adding a minor tweak to the wrapfile.

Also update the revision to HEAD rather than pinning.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iddbbf8117d7d8e7a0cc6a4f9ad5c6e9bff984834
diff --git a/subprojects/nlohmann-json.wrap b/subprojects/nlohmann-json.wrap
index a77b7c9..3745380 100644
--- a/subprojects/nlohmann-json.wrap
+++ b/subprojects/nlohmann-json.wrap
@@ -1,3 +1,6 @@
 [wrap-git]
-revision = b3e5cb7f20dcc5c806e418df34324eca60d17d4e
+revision = HEAD
 url = https://github.com/nlohmann/json.git
+
+[provide]
+nlohmann_json = nlohmann_json_dep