internal: Add missing Manager forward declaration

This allows us to not depend on specific include ordering.

Change-Id: I4d2a552eb4b4d5dcf0aaf9ad9be3b86265ca469a
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/dump_internal.hpp b/dump_internal.hpp
index d13ae24..6ef0fa0 100644
--- a/dump_internal.hpp
+++ b/dump_internal.hpp
@@ -9,6 +9,8 @@
 {
 namespace dump
 {
+
+class Manager;
 namespace internal
 {