[gnome-builder] cargo: add include for Makefile.plugin

[Christian Hergert <chergert src gnome org>]

commit 5893c397866b3c5db641e4299cb819e313dbefc0
Author: Christian Hergert <chergert redhat com>
Date:   Sun Oct 30 02:14:26 2016 -0700

    cargo: add include for Makefile.plugin

 plugins/cargo/Makefile.am |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/plugins/cargo/Makefile.am b/plugins/cargo/Makefile.am
index 1c801a4..6ed1f1b 100644
--- a/plugins/cargo/Makefile.am
+++ b/plugins/cargo/Makefile.am
@@ -9,4 +9,6 @@ dist_plugin_DATA = \
 
 endif
 
+include $(top_srcdir)/plugins/Makefile.plugin
+
 -include $(top_srcdir)/git.mk