msc/gsup_client_mux.h: remove include of itself

Change-Id: I89ffdb98e98133bd28023a19076f3a5f02e520d2
diff --git a/include/osmocom/msc/gsup_client_mux.h b/include/osmocom/msc/gsup_client_mux.h
index 501b81d..f327786 100644
--- a/include/osmocom/msc/gsup_client_mux.h
+++ b/include/osmocom/msc/gsup_client_mux.h
@@ -1,7 +1,6 @@
 #pragma once
 
 #include <osmocom/gsm/gsup.h>
-#include <osmocom/msc/gsup_client_mux.h>
 
 struct gsup_client_mux;
 struct ipaccess_unit;