AIX doesn't have tm_gmtoff

[ayappanec]

AIX doesn't have tm_gmtoff due to which the compilation is failing. This PR will fix this issue.

[codecov-io]

Codecov Report

Merging #876 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #876   +/-   ##
=======================================
  Coverage   90.09%   90.09%           
=======================================
  Files          71       71           
  Lines        3271     3271           
=======================================
  Hits         2947     2947           
  Misses        324      324

Impacted Files	Coverage Δ
src/Date.cpp	69.4% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f3c5a34...efcd91e. Read the comment docs.

[eddelbuettel]

Hm, we have not had any AIX reports in some time. I wonder why that did not come up before. I see two other files with five total uses for #ifdef _AIX so this will probably not cause harm -- but still strange we did not need it before.

[ayappanec]

Not sure. I am just trying R in AIX and i found this issue.