Fixes the generated path of signed_key.proto

BUG=None
TBR=oshima@chromium.org
TEST=None

Review URL: https://codereview.chromium.org/649803002

Cr-Commit-Position: refs/heads/master@{#299187}

chromeos/BUILD.gn

@@ -189,5 +189,5 @@ proto_library("cryptohome_signkey_proto") {
     "//third_party/cros_system_api/dbus/cryptohome/signed_secret.proto",
   ]
 
-  proto_out_dir = "chromeos/dbus/cryptohome"
+  proto_out_dir = "chromeos/cryptohome"
 }