Fix check for missing goog.require directives

Take this example:
```
goog.require('ol.projection.addCommonProjections');

ol.projection.addCommonProjections();
```

Without this patch, the build_check_requires_timestamp target would report
that ``ol.projection`` is missing.
This is because ``ol.projection.addCommonProjections();`` would match both
``ol.projection.addCommonProjections`` and ``ol.projection`` ``goog.provide``
directives.