debian: Fix the installation of the sgsn example data

Creating the sgsn package failed because the debian directory has
been moved and the .examples file was not updated. This was reported
by plotr.
diff --git a/debian/osmocom-sgsn.examples b/debian/osmocom-sgsn.examples
index 5cc4db5..15de78d 100644
--- a/debian/osmocom-sgsn.examples
+++ b/debian/osmocom-sgsn.examples
@@ -1 +1 @@
-doc/examples/osmo-sgsn
+openbsc/doc/examples/osmo-sgsn