fix(Dropdown): fix warning on PropTypes with React >= 15.5.0

[g1eny0ung]

Fixes #1587

Seems that the commit 3a09c0f missed a PropTypes, so it also has a warning when in developing.

[codecov-io]

Codecov Report

Merging #1588 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1588   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files         141      141           
  Lines        2384     2384           
=======================================
  Hits         2378     2378           
  Misses          6        6

Impacted Files	Coverage Δ
src/modules/Dropdown/Dropdown.js	100% <ø> (ø)	⬆️

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[layershifter]

@g1eny0ung Thanks for PR. I overlooked this in #1576

[g1eny0ung]

@layershifter 👌🏽

[levithomason]

Thanks!

[rafaelkallis]

@levithomason When can we expect the next release including this PR?

[levithomason]

I release typically about once a week, usually on weekends or Monday. Just released 0.68.0 2 days ago. If there are high priority items, I'll release more often. Since this is just an annoying warning, it'll probably go out this weekend.

[levithomason]

Released in semantic-ui-react@0.68.1.