define stream_security_context_t as a struct to get type checking

By using a struct we may have warnings at compilation time in some
cases if the type is not used properly.

The type definition becomes a bit weird, so we need to properly
comment.

If too strange, we can revert to the classical "void" definition.

openair3/SECU/secu_defs.h

@@ -28,10 +28,14 @@
 #define SECU_DIRECTION_UPLINK   0
 #define SECU_DIRECTION_DOWNLINK 1
 
-/* stream_security_context_t is an opaque structure
- * it is different for each integrity and ciphering algorithm in use
+/* stream_security_context_t is an opaque structure.
+ * It is different for each integrity and ciphering algorithm in use.
+ * Defined as a struct to have compilation time type checking
+ * (the "context" field is never actually used).
  */
-typedef void stream_security_context_t;
+typedef struct {
+  void *context;
+} stream_security_context_t;
 
 /* stream_security_container_t contains the current configuration
  * of integrity and ciphering. It is used by PDCP and NAS.