Add -f flag to invocation of rm in clean script

### Why?

If the `include` directory doesn't exist (probably because nobody ran
`yarn headers`) then attempting to remove it will fail.
This isn't useful for a `clean` command: if the directory is missing
then it's already clean!

### What?

* add `-f` to `rm` command

package.json

@@ -23,7 +23,7 @@
   },
   "scripts": {
     "build": "node-gyp rebuild",
-    "clean": "rm -r build include",
+    "clean": "rm -rf build include",
     "headers": "node-gyp install --devdir include",
     "test": "jest"
   },