add to dist: doc subdir with all current doc files

Change-Id: Idfef4f050e21a5d0eae3a530fe319a5adf81534d
diff --git a/configure.ac b/configure.ac
index 38e3b4c..2ca32b1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -42,4 +42,5 @@
 	include/osmocom/ranap/Makefile
 	include/osmocom/rua/Makefile
 	include/osmocom/iuh/Makefile
+	doc/Makefile
 	)