Update cub submodule again (fixes GPU build)

[RAMitchell]

Add fix from NVlabs/cub#100

[codecov-io]

Codecov Report

Merging #2599 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2599      +/-   ##
==========================================
+ Coverage   35.11%   35.14%   +0.02%     
==========================================
  Files          79       79              
  Lines        6972     6972              
  Branches      680      680              
==========================================
+ Hits         2448     2450       +2     
  Misses       4424     4424              
+ Partials      100       98       -2

Impacted Files	Coverage Δ
src/objective/regression_obj.cc	86.33% <0%> (-1.1%)	⬇️
src/metric/rank_metric.cc	91.35% <0%> (+0.54%)	⬆️
src/data/simple_dmatrix.cc	57.33% <0%> (+0.66%)	⬆️
src/data/sparse_page_dmatrix.cc	87.97% <0%> (+1.09%)	⬆️

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