vm: Fix declaration settings (ethereumjs#1999)

Author: acolytec3

packages/vm/tsconfig.benchmarks.json

@@ -3,7 +3,7 @@
   "compilerOptions": {
     "lib": ["dom"],
     "sourceMap": false,
-    "declaration": false,
+    "declaration": true,
     "esModuleInterop": true
   },
   "include": ["benchmarks/**.ts"]