[R] remove warning in configure.ac (fixes #6151)

[jameslamb]

This PR proposes a fix for #6151 . I think it will resolve the WARNING on https://cran.r-project.org/web/checks/check_results_xgboost.html

Version: 1.2.0.1
Check: whether package can be installed
Result: WARN
    Found the following significant warnings:
     WARNING: OpenMP is unavailable on this Mac OSX system. Training speed may be suboptimal.
Flavor: r-release-macos-x86_64

[jameslamb]

oh sorry forgot to update configure! will do that right now

[codecov-commenter]

Codecov Report

Merging #6152 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6152   +/-   ##
=======================================
  Coverage   78.83%   78.83%           
=======================================
  Files          12       12           
  Lines        3090     3090           
=======================================
  Hits         2436     2436           
  Misses        654      654           

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