[build] Add missing deps definition

This fixes MSVC Windows builds that were broken due to a missing deps
definition.

Bug: v8:9954
Change-Id: I19c5112226caadae6a0221acee7bcf19cf0abbc8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1906379
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64873}

BUILD.gn

@@ -1351,6 +1351,7 @@ if (!v8_use_external_startup_data) {
       "//:gn_visibility",
     ]
 
+    deps = []
     public_deps = [
       # This should be public so downstream targets can declare the snapshot
       # output file as their inputs.