build: Update sdbusplus-gen-meson

Change-Id: Idb647ddb3b0453d05cf3a55e883e0f4b26227b81
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/meson.build b/meson.build
index f227b1a..36b00f6 100644
--- a/meson.build
+++ b/meson.build
@@ -26,6 +26,10 @@
 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
 
 generated_sources = []
 generated_others = []