[build] Fix build dependencies for embedded snapshot

The external snapshot target is missing a few build
dependencies.

TBR=vogelheim@chromium.org, yangguo@chromium.org, clemensh@chromium.org

Review URL: https://codereview.chromium.org/1904013002

Cr-Commit-Position: refs/heads/master@{#35686}

tools/gyp/v8.gyp

@@ -299,6 +299,8 @@
               'action_name': 'run_mksnapshot (external)',
               'inputs': [
                 '<(mksnapshot_exec)',
+                '<(embed_script)',
+                '<(warmup_script)',
               ],
               'variables': {
                 'mksnapshot_flags': [],