Update nlohmann-json subproject handling
Without this, a build with subprojects would fail with:
```
meson.build:89:24: ERROR: Include dir subprojects/nlohmann-json/single_include does not exist.
```
Change-Id: I26e26a7a0473ffb163fc4ca9f1ac291f13de261e
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/subprojects/nlohmann-json.wrap b/subprojects/nlohmann-json.wrap
index a77b7c9..a2884b1 100644
--- a/subprojects/nlohmann-json.wrap
+++ b/subprojects/nlohmann-json.wrap
@@ -1,3 +1,6 @@
[wrap-git]
-revision = b3e5cb7f20dcc5c806e418df34324eca60d17d4e
-url = https://github.com/nlohmann/json.git
+url = https://github.com/nlohmann/json
+revision = HEAD
+
+[provide]
+nlohmann-json = nlohmann_json_dep