[fuchsia] Add fuchsia.netstack to public_deps.

NetworkChangeNotifierFuchsia #includes the fuchsia.netstack FIDL header
in a header that is published by the //net target, requiring that the
fuchsia.netstack dependency is itself public.

TBR=mmenke

Bug: b/149757507, 1050703
Change-Id: I6bc7088817fd0180b1da85d3c40c0ca7a548fc1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066719Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743186}

net/BUILD.gn

@@ -1456,6 +1456,7 @@ component("net") {
     }
 
     if (is_fuchsia) {
+      public_deps += [ "//third_party/fuchsia-sdk/sdk:fuchsia-netstack" ]
       sources += [
         "base/network_change_notifier_fuchsia.cc",
         "base/network_change_notifier_fuchsia.h",