[build] Add missing library dependencies to BUILD.gn

These libraries' headers are used directly by the code, but the
libraries were previously depended on only indirectly via other
libraries that happen to use them.

Bug: BLD-437 #comment Missing deps blocking //zircon/public/lib cleanup
Change-Id: If34138b6902dd6a512374264131477e5f4403d3f
diff --git a/BUILD.gn b/BUILD.gn
index a533df2..98baa5e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -54,10 +54,11 @@
     "fuchsia/fuchsia-compat.h",
   ]
   public_deps = [
+    "//zircon/public/fidl/fuchsia-hardware-pty:fuchsia-hardware-pty_c",
     "//zircon/public/lib/async",
+    "//zircon/public/lib/async-loop",
     "//zircon/public/lib/fdio",
     "//zircon/public/lib/loader-service",
-    "//zircon/public/fidl/fuchsia-hardware-pty:fuchsia-hardware-pty_c",
   ]
   configs += [ ":fuchsia" ]
 }