Update build.gradle - added namespace to for gradle 8.0 and above

[ettaegbe]

A problem occurred configuring project ':libphonenumber_plugin'.
  Could not create an instance of type com.android.build.api.variant.impl.LibraryVariantBuilderImpl.
     Namespace not specified. Please specify a namespace in the module's build.gradle file like so:
      android {
          namespace 'com.example.namespace'
      }