[fs_management] Fix use of syslog

Fixed a crash when using FX_LOGS in the c++ fs_management library.

Fixed: 93586
Change-Id: I5e7fa7c6e21cc614b1bd2b342caed42ee3f13795
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/679865
Fuchsia-Auto-Submit: Chris Suter <csuter@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Chris Drouillard <cdrllrd@google.com>

src/lib/storage/fs_management/cpp/BUILD.gn

@@ -48,6 +48,7 @@ if (is_fuchsia) {
       "//sdk/fidl/fuchsia.hardware.block.partition:fuchsia.hardware.block.partition_c",
       "//sdk/fidl/fuchsia.hardware.block.volume:fuchsia.hardware.block.volume_c",
       "//sdk/fidl/fuchsia.hardware.block.volume:fuchsia.hardware.block.volume_llcpp",
+      "//sdk/lib/syslog/cpp:backend_legacy",
       "//src/lib/digest",
       "//src/lib/storage/block_client/cpp",
       "//src/lib/storage/vfs/cpp",