This avoids unnecessarily embedding dllexport directives in all
the object files.

This also fixes building with clang in MSVC mode, as the MSVC
definition of XSLTPUBVAR in xsltexports.h is lacking 'extern'
when doing dllexport. (This probably used to work before
compilers defaulted to -fcommon.) This fixes duplicate symbols
like these:

lld-link: error: duplicate symbol: xsltGenericError
>>> defined at ../src/libs/xslt/libxslt/xsltutils.h:140
>>>            libxslt.cross.a(documents.cross.o)
>>> defined at ../src/libs/xslt/libxslt/xsltutils.h:140
>>>            libxslt.cross.a(extensions.cross.o)
Signed-off-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>