babeld: Include babel_main.h in noinst_HEADERS.

babeld wouldn't build in a dist tarball without this.

From: Denis Ovsienko <infrastation@yandex.ru>
Signed-off-by: Juliusz Chroboczek <jch@pps.jussieu.fr>
[fixed up git message]
Signed-off-by: David Lamparter <equinox@diac24.net>
diff --git a/babeld/Makefile.am b/babeld/Makefile.am
index 468b5a5..8703de0 100644
--- a/babeld/Makefile.am
+++ b/babeld/Makefile.am
@@ -18,7 +18,7 @@
 noinst_HEADERS = \
 	babel_zebra.h net.h kernel.h util.h source.h neighbour.h	\
 	route.h xroute.h message.h resend.h babel_interface.h babeld.h	\
-	babel_filter.h
+	babel_filter.h babel_main.h
 
 babeld_SOURCES = \
 	babel_main.c $(libbabel_a_SOURCES)