meson: add nlohmann-json wrapper and deps

A number of locations within the repository do not appropriately
specify the dependency on nlohmann-json.  Fix up the wrap file,
add appropriate dependency detection in the root meson, and add the
library as a dependency when needed.  This gets closer to allowing
full subproject-based builds.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If70888c6de5651ca355f84616e41408ddde9fc08
diff --git a/user_channel/meson.build b/user_channel/meson.build
index 36ea83b..eec8b82 100644
--- a/user_channel/meson.build
+++ b/user_channel/meson.build
@@ -5,6 +5,7 @@
   dependencies: [
     crypto,
     ipmid_dep,
+    nlohmann_json_dep,
     phosphor_dbus_interfaces_dep,
     phosphor_logging_dep,
     std_cpp_fs,
@@ -38,9 +39,10 @@
       channellayer_dep,
       crypto,
       ipmid_dep,
+      nlohmann_json_dep,
+      pam,
       phosphor_dbus_interfaces_dep,
       phosphor_logging_dep,
-      pam,
       std_cpp_fs,
       systemd,
     ])