CR: 8180540: Add pandoc build fix for windows

[Brad R. Wetmore]

Magnus,

I've added your suggested fix to spec.gmk.in, which is the minor tweak 
to add @FIXPATH@ to allow pandoc to run on windows builds.

    https://bugs.openjdk.java.net/browse/JDK-8180540
    http://cr.openjdk.java.net/~wetmore/8180540/webrev.00/

I put you down as Contributed-by: with myself as reviewer, but thought 
you (and others?) might want to at least check it.

Thanks,

Brad