Fix typo in stream_init_message.hh

The debug build uncovered this typo. It was setting a class member with
itself (with an undefined value) instead from the parameter, which I was
surprised the compiler didn't catch at compile time.

Discovered in issue #368.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>

streaming/messages/stream_init_message.hh

@@ -72,7 +72,7 @@ public:
         , plan_id(_plan_id)
         , description(_description)
         , is_for_outgoing(_is_for_outgoing)
-        , keep_ss_table_level(-keep_ss_table_level) {
+        , keep_ss_table_level(_keep_ss_table_level) {
     }
 
 #if 0