Fix check for unused goog.require directives

The ``build_check_requires_timestamp`` target was broken since 14e76b3a11653191959e63c3c9bc3e4d0083e29e.
A ``goog.require('my.require');`` line was counted in both ways:
 * that ``my.require`` is stated as a requirement,
 * and that this ``my.require`` requirement is actually used in the code.
so the check for unused requires was always returning 0...