debian: Add libosmoctrl-doc sub-package

This fixes the debian package builds for libosmocore.git

Change-Id: I454e3c0175ab5a89e8925e036a3644299367df0f
diff --git a/debian/libosmoctrl-doc.install b/debian/libosmoctrl-doc.install
new file mode 100644
index 0000000..c7afaf1
--- /dev/null
+++ b/debian/libosmoctrl-doc.install
@@ -0,0 +1 @@
+usr/share/doc/libosmocore/ctrl/