gen: update due to sdbusplus-gen-meson change

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I7dfb64d1567c829fe5a3ac71bf5c9aff6773abc7
diff --git a/meson.build b/meson.build
index 86d0499..b58fc85 100644
--- a/meson.build
+++ b/meson.build
@@ -18,6 +18,10 @@
 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
 
 pdi_dep = dependency('phosphor-dbus-interfaces')