doxygen: fix correct use of doc/core subdirectory
diff --git a/Makefile.am b/Makefile.am
index 6cb39ea..64f13dd 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -27,7 +27,7 @@
 		  $(top_builddir)/doc/gsm/html/index.html \
 		  $(top_builddir)/doc/vty/html/index.html \
 		  $(top_builddir)/doc/codec/html/index.html
-	cd $(top_builddir)/doc && tar cf html.tar *
+	cd $(top_builddir)/doc && tar cf html.tar */html
 
 $(top_builddir)/doc/core/html/index.html: $(SOURCES) Doxyfile
 	rm -rf doc/core