Symlink equivs-generated "ax-debian" as "debian/control"

This allows usage of some standard Debian package management commands
like dch(1), for example.

diff --git a/debian/control b/debian/control
new file mode 120000 (symlink)
index 0000000..73b3fe7
--- /dev/null
+++ b/debian/control
@@ -0,0 +1 @@
+../ax-debian
\ No newline at end of file