Add //printing:printing as a traits deps to printing_backend_mojo

Currently print_backend_mojom_traits.h includes printing/backend/print_backend.h"
so without this deps this target can't be built by itself.

TBR=awscreen@chromium.org

Bug: 1135263
Change-Id: Ic597468dcdaefbb83e145ef82bb4f4d6b93ef6c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2449916Reviewed-by: Chris Sharp <csharp@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813908}

printing/backend/mojom/BUILD.gn

@@ -19,6 +19,7 @@ mojom("mojom") {
       ]
       traits_sources = [ "print_backend_mojom_traits.cc" ]
       traits_headers = [ "print_backend_mojom_traits.h" ]
+      traits_deps = [ "//printing:printing" ]
     },
   ]