build: Add headers to dependency

When we don't have this specified, dependant source code can sometimes
get built in parallel prior to the headers being generated and
compilation will fail.

Change-Id: I1e165adfb86e0978fdbf143913328ca83327c497
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/meson.build b/meson.build
index b812541..759d836 100644
--- a/meson.build
+++ b/meson.build
@@ -135,6 +135,7 @@
 )
 
 phosphor_dbus_interfaces_dep = declare_dependency(
+    sources: generated_files_headers,
     include_directories: include_directories('gen'),
     link_with: libphosphor_dbus,
     dependencies: sdbusplus_dep,