gen: update due to sdbusplus-gen-meson change

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I9491eb608d9c6849b3e39fe212054a3651bfb824
diff --git a/meson.build b/meson.build
index 8bba339..70cd52a 100644
--- a/meson.build
+++ b/meson.build
@@ -13,6 +13,10 @@
 sdbusplus_dep = dependency('sdbusplus')
 sdbusplusplus_prog = find_program('sdbus++', native: true)
 sdbuspp_gen_meson_prog = find_program('sdbus++-gen-meson', native: true)
+sdbusplusplus_depfiles = files()
+if sdbusplus_dep.type_name() == 'internal'
+    sdbusplusplus_depfiles = subproject('sdbusplus').get_variable('sdbusplusplus_depfiles')
+endif
 
 # Parse options to determine appropriate subdirectories to support.
 selected_subdirs = []