fix: include proto d.ts in the build (#335)

Author: JustinBeckwith

package.json

@@ -10,6 +10,7 @@
   "repository": "googleapis/nodejs-datastore",
   "main": "./build/src/index.js",
   "files": [
+    "build/proto",
     "build/protos",
     "build/src"
   ],
@@ -35,7 +36,7 @@
     "system-test": "mocha build/system-test --timeout 600000",
     "fix": "gts fix && eslint '**/*.js' --fix",
     "clean": "gts clean",
-    "compile": "tsc -p . && cp -r src/v1 build/src && cp -r protos build && cp test/*.js build/test",
+    "compile": "tsc -p . && cp -r src/v1 build/src && cp -r proto* build && cp test/*.js build/test",
     "prepare": "npm run compile",
     "pretest": "npm run compile",
     "proto": "npm run proto:datastore",