meson: adjust nlohmann-json dependency

Simplify the meson dependency logic and reuse the subproject wrap from
other repositories for consistency.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I74646501b1e10c01cb2af47c294fcf5a8a89e7c3
diff --git a/src/meson.build b/src/meson.build
index 9529d42..ee9778d 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -24,10 +24,8 @@
   main_srcs += [
     'inventory_mac.cpp',
   ]
-  # nlohmann_json might not have a pkg-config. It is header only so just make
-  # sure we can access the needed symbols from the header.
   main_deps += [
-    dependency('nlohmann_json', allow_fallback: true, required: false),
+    dependency('nlohmann_json', include_type: 'system')
   ]
 endif