target: avoid using interp global variable

Adds 'interp' to target_event_action structure to avoid using the
global variable of the same name.

src/target/target.h

@@ -222,6 +222,7 @@ enum target_event
 
 struct target_event_action {
 	enum target_event event;
+	Jim_Interp *interp;
 	struct Jim_Obj *body;
 	int has_percent;
 	struct target_event_action *next;