debian: Add libosmoctrl-doc sub-package

This fixes the debian package builds for libosmocore.git

Change-Id: I454e3c0175ab5a89e8925e036a3644299367df0f
diff --git a/debian/rules b/debian/rules
index 2399b01..6eb7346 100755
--- a/debian/rules
+++ b/debian/rules
@@ -52,6 +52,7 @@
 	$(RM) tests/testsuite
 	$(RM) -r doc/codec/
 	$(RM) -r doc/core/
+	$(RM) -r doc/ctrl/
 	$(RM) -r doc/gsm/
 	$(RM) -r doc/gb/
 	$(RM) -r doc/vty/html/