[GHC] #16387: Duplicate vsnwprintf symbol on Windows

[GHC]

#16387: Duplicate vsnwprintf symbol on Windows
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        Reporter:  bgamari       |                Owner:  (none)
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:
       Component:  Compiler      |              Version:  8.6.3
      Resolution:                |             Keywords:  broken-test
Operating System:  Windows       |         Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Comment (by Phyx-):

 I'll need to double check but I believe we always `libmingwex` now? If so
 we can just remove the built-in symbol or mark it weak.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16387#comment:6>
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