Move out sgsn to its own subdir

Change-Id: I16fccc0eadf588599b9e5578d0f4dbaf9df81737
diff --git a/configure.ac b/configure.ac
index 6672204..ec7af6e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -247,6 +247,7 @@
     include/osmocom/sgsn/Makefile
     src/Makefile
     src/gprs/Makefile
+    src/sgsn/Makefile
     src/gbproxy/Makefile
     src/gtphub/Makefile
     tests/Makefile