Package Kit: Fix -Wmismatched-tags

Replace class with struct, since BJobStateListener is previously declared
as a struct in Job.h
Pointed by clang  [-Wmismatched-tags]

Change-Id: I75293b48bcb521d25e98702d1fc1e0ab1008d504
Reviewed-on: https://review.haiku-os.org/482
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

headers/os/package/Context.h

@@ -11,7 +11,7 @@
 
 
 namespace BSupportKit {
-	class BJobStateListener;
+	struct BJobStateListener;
 }