Add autogen.sh to distrib

For https://bugzilla.gnome.org/show_bug.cgi?id=761782

autogen.sh was not included in the tarball releases which complicates
situations when users have to make changes to e.g. configure.ac andi
have to re-generate configure.
diff --git a/Makefile.am b/Makefile.am
index 70720f3..cbee997 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1207,7 +1207,7 @@
 	     check-xsddata-test-suite.py check-xinclude-test-suite.py \
              example/Makefile.am example/gjobread.c example/gjobs.xml \
 	     $(man_MANS) libxml-2.0.pc.in libxml-2.0-uninstalled.pc.in \
-	     libxml2-config.cmake.in \
+	     libxml2-config.cmake.in autogen.sh \
 	     trionan.c trionan.h triostr.c triostr.h trio.c trio.h \
 	     triop.h triodef.h libxml.h elfgcchack.h xzlib.h buf.h \
 	     enc.h save.h testThreadsWin32.c genUnicode.py TODO_SCHEMAS \