Get rid of a few trivial compiler warnings.

[omo]

Mostly about adjusting types.

[codecov-io]

Codecov Report

Merging #4312 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4312      +/-   ##
==========================================
+ Coverage   67.82%   67.82%   +<.01%     
==========================================
  Files         132      132              
  Lines       12201    12200       -1     
==========================================
  Hits         8275     8275              
+ Misses       3926     3925       -1

Impacted Files	Coverage Δ
src/tree/updater_colmaker.cc	39.76% <ø> (+0.19%)	⬆️
src/tree/updater_basemaker-inl.h	63.59% <ø> (-0.18%)	⬇️
src/c_api/c_api.cc	23.53% <100%> (ø)	⬆️
src/data/sparse_page_dmatrix.cc	100% <100%> (ø)	⬆️

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[omo]

Thanks!