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/meson.build b/meson.build
index de6feed..048cbce 100644
--- a/meson.build
+++ b/meson.build
@@ -130,11 +130,24 @@
         ],
     )
 
+    if cxx.has_header('nlohmann/json.hpp')
+        nlohmann_json_dep = declare_dependency()
+    else
+        subproject('nlohmann-json')
+        nlohmann_json_dep = declare_dependency(
+            include_directories: [
+                'subprojects/nlohmann-json/single_include',
+                'subprojects/nlohmann-json/single_include/nlohmann',
+            ]
+        )
+    endif
+
     deps += [
+        nlohmann_json_dep,
+        phosphor_dbus_interfaces_dep,
+        phosphor_logging_dep,
         sdbusplus_dep,
         sdeventplus_dep,
-        phosphor_logging_dep,
-        phosphor_dbus_interfaces_dep,
     ]
 
     sources += [