bench: add missing require back

Signed-off-by: Philipp Burckhardt <pburckhardt@outlook.com>

Author: Planeshifter

lib/node_modules/@stdlib/stats/base/dists/negative-binomial/variance/benchmark/benchmark.js

@@ -25,6 +25,7 @@ var uniform = require( '@stdlib/random/base/uniform' );
 var Float64Array = require( '@stdlib/array/float64' );
 var discreteUniform = require( '@stdlib/random/base/discrete-uniform' );
 var isnan = require( '@stdlib/math/base/assert/is-nan' );
+var EPS = require( '@stdlib/constants/float64/eps' );
 var pkg = require( './../package.json' ).name;
 var variance = require( './../lib' );