[react-tools] Add src/ to files

This is so it's possible to use the original @providesModule source
files in a toolchain that understands those.

package.json

@@ -20,6 +20,7 @@
     "main.js",
     "bin/jsx",
     "build/modules/",
+    "src/",
     "vendor/fbtransform/",
     "vendor/constants.js"
   ],