[fuchsia] Add ledger to the allow list.

Change-Id: Iceb174dbb8ee8f66f84292312256af3c2791170a

BUILD.gn

@@ -6,6 +6,7 @@
 # abseil-cpp from the Fuchsia platform source tree once cobalt
 # moves out-of-tree.
 visibility = [
+  "//src/ledger/*",
   "//third_party/abseil-cpp/*",
   "$cobalt_root/*",
   "//third_party/tink/*",