configure/Makefile: Include contrib in "make dist"

Change-Id: I2d6a2e06f075391e8e51d1342a1afe955cc127b0
diff --git a/configure.ac b/configure.ac
index c4d9766..d2c5e58 100644
--- a/configure.ac
+++ b/configure.ac
@@ -169,6 +169,9 @@
 AC_OUTPUT(
 	Makefile
 	libosmo-rspro.pc
+	contrib/Makefile
+	contrib/etc_default/Makefile
+	contrib/systemd/Makefile
 	doc/Makefile
 	doc/manuals/Makefile
 	src/Makefile