[ledger] Breaking out transitive #include of scoped_callback.h

This is to support https://fuchsia-review.googlesource.com/c/garnet/+/229494,
wherein scoped_task_runner.h no longer #includes scoped_callback.h.

Test: fx build (with and without garnet/229494)
Change-Id: I50750fb89303719cc821c146ca731ad9833114ad
diff --git a/bin/ledger/app/ledger_manager.cc b/bin/ledger/app/ledger_manager.cc
index e9ee3b4..8f4efa0 100644
--- a/bin/ledger/app/ledger_manager.cc
+++ b/bin/ledger/app/ledger_manager.cc
@@ -8,6 +8,7 @@
 #include <utility>
 #include <vector>
 
+#include <lib/callback/scoped_callback.h>
 #include <lib/fidl/cpp/interface_request.h>
 #include <lib/fit/defer.h>
 #include <lib/fit/function.h>
diff --git a/bin/ledger/app/merging/auto_merge_strategy.cc b/bin/ledger/app/merging/auto_merge_strategy.cc
index 1c03612..071d5d2 100644
--- a/bin/ledger/app/merging/auto_merge_strategy.cc
+++ b/bin/ledger/app/merging/auto_merge_strategy.cc
@@ -8,6 +8,7 @@
 #include <string>
 #include <vector>
 
+#include <lib/callback/scoped_callback.h>
 #include <lib/fit/function.h>
 #include <lib/fxl/memory/ref_ptr.h>
 #include <lib/fxl/memory/weak_ptr.h>
diff --git a/bin/ledger/app/merging/conflict_resolver_client.cc b/bin/ledger/app/merging/conflict_resolver_client.cc
index e01a8df..469f5a7 100644
--- a/bin/ledger/app/merging/conflict_resolver_client.cc
+++ b/bin/ledger/app/merging/conflict_resolver_client.cc
@@ -9,6 +9,7 @@
 #include <string>
 #include <vector>
 
+#include <lib/callback/scoped_callback.h>
 #include <lib/callback/waiter.h>
 #include <lib/fit/function.h>
 #include <lib/fsl/socket/strings.h>
diff --git a/bin/ledger/app/merging/last_one_wins_merge_strategy.cc b/bin/ledger/app/merging/last_one_wins_merge_strategy.cc
index 149922d..da56323 100644
--- a/bin/ledger/app/merging/last_one_wins_merge_strategy.cc
+++ b/bin/ledger/app/merging/last_one_wins_merge_strategy.cc
@@ -7,6 +7,7 @@
 #include <memory>
 #include <string>
 
+#include <lib/callback/scoped_callback.h>
 #include <lib/callback/waiter.h>
 #include <lib/fit/function.h>
 #include <lib/fxl/memory/ref_ptr.h>