share chapters/gsup.adoc from OsmoSGSN to OsmoMSC + OsmoHLR

Since the NITB split, GSUP is used in all three network elements, so
make the protocol a shared chapter

Change-Id: Id2d7c27ef16eb0ebe5f60d625a1fcf42f1603f4f
diff --git a/doc/manuals/osmomsc-usermanual.adoc b/doc/manuals/osmomsc-usermanual.adoc
index 6190481..ac68d93 100644
--- a/doc/manuals/osmomsc-usermanual.adoc
+++ b/doc/manuals/osmomsc-usermanual.adoc
@@ -28,6 +28,8 @@
 
 include::../common/chapters/control_if.adoc[]
 
+include::../common/chapters/gsup.adoc[]
+
 include::../common/chapters/port_numbers.adoc[]
 
 include::../common/chapters/bibliography.adoc[]