WTF.gypi lists Platform.h twice
https://bugs.webkit.org/show_bug.cgi?id=105731

Reviewed by Tony Chang.

Once ought to be enough for everybody.

* WTF.gypi:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@138628 268f45cc-cd09-0410-ab3c-d52691b4dbfc