meson: add dependency for nlohmann-json

The code uses nlohmann-json but didn't have a dependency.  Add one
so that the project builds in subproject builds.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ic28b50ee888f909a07833eb57f9be13f4b7bf6d5
diff --git a/subprojects/nlohmann-json.wrap b/subprojects/nlohmann-json.wrap
new file mode 100644
index 0000000..9096612
--- /dev/null
+++ b/subprojects/nlohmann-json.wrap
@@ -0,0 +1,3 @@
+[wrap-git]
+url = https://github.com/nlohmann/json.git
+revision = HEAD