[terminal_settings_agent] Update deps on fidl from garnet to src

This CL updates the terminal_settings_agent BUILD.gn to the new location
of the FIDL Rust library. It was moved from garnet/ to src/ in
https://fuchsia-review.googlesource.com/c/fuchsia/+/355373.

Test: no behavior change
Change-Id: I72cf8b61c983cddba2b819173a22a72bfafc4186
diff --git a/bin/terminal_settings_agent/BUILD.gn b/bin/terminal_settings_agent/BUILD.gn
index 884539c..4657714 100644
--- a/bin/terminal_settings_agent/BUILD.gn
+++ b/bin/terminal_settings_agent/BUILD.gn
@@ -23,9 +23,9 @@
 
   deps = [
     "//garnet/lib/rust/io_util",
-    "//garnet/public/lib/fidl/rust/fidl",
     "//sdk/fidl/fuchsia.terminal:fuchsia.terminal-rustc",
     "//sdk/fidl/fuchsia.ui.types:fuchsia.ui.types-rustc",
+    "//src/lib/fidl/rust/fidl",
     "//src/lib/fuchsia-async",
     "//src/lib/fuchsia-component",
     "//src/lib/syslog/rust:syslog",