fix: adds node-pre-gyp as dev dependency (open-telemetry#322)

Author: bg451

packages/opentelemetry-plugin-grpc/package.json

@@ -47,6 +47,7 @@
     "gts": "^1.1.0",
     "mocha": "^6.2.0",
     "nyc": "^14.1.1",
+    "node-pre-gyp": "^0.12.0",
     "tslint-microsoft-contrib": "^6.2.0",
     "tslint-consistent-codestyle":"^1.15.1",
     "ts-mocha": "^6.0.0",