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

[Christian Hergert <chergert src gnome org>]

commit c8a22702e7e3ddfbab707805940838e63d200c34
Author: Christian Hergert <chergert redhat com>
Date:   Sun Oct 30 02:14:33 2016 -0700

    rust-langserv: add include for Makefile.plugin

 plugins/rust-langserv/Makefile.am |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/plugins/rust-langserv/Makefile.am b/plugins/rust-langserv/Makefile.am
index 33946f4..5dd7d88 100644
--- a/plugins/rust-langserv/Makefile.am
+++ b/plugins/rust-langserv/Makefile.am
@@ -9,4 +9,6 @@ dist_plugin_DATA = \
 
 endif
 
+include $(top_srcdir)/plugins/Makefile.plugin
+
 -include $(top_srcdir)/git.mk