build: add v8_context_snapshot_generator to mksnapshot zip (#15606)

v8_context_snapshot_generator is needed when creating custom snapshots with mksnapshot

BUILD.gn

@@ -912,6 +912,7 @@ dist_zip("electron_chromedriver_zip") {
 dist_zip("electron_mksnapshot_zip") {
   data_deps = [
     "//v8:mksnapshot($v8_snapshot_toolchain)",
+    "//tools/v8_context_snapshot:v8_context_snapshot_generator",
     ":licenses",
   ]
   outputs = [