registry: add registry_target to the external dependency

When PDI is used as a subproject, we need some dependency that indicates
the registry is populated.  Add the internal build rule to the primary
external dependency so any dependency on PDI satisfies the registry
population.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Id16b4c272a82f5451e58b271e02a622b108ffaa3
diff --git a/meson.build b/meson.build
index a167846..0fc0644 100644
--- a/meson.build
+++ b/meson.build
@@ -96,7 +96,7 @@
 )
 
 phosphor_dbus_interfaces_dep = declare_dependency(
-    sources: generated_headers,
+    sources: [generated_headers, registry_target],
     include_directories: include_directories('gen'),
     link_with: libphosphor_dbus,
     dependencies: sdbusplus_dep,