[libxslt] Fix export of xsltExtMarker on Windows

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 0141e19eef59863ceac7fcb00345bc9432e3bf11
Author: David Kilzer <ddkilzer apple com>
Date:   Fri Jan 24 13:46:33 2020 -0800

    Fix export of xsltExtMarker on Windows
    
    * libxslt/preproc.h:
    (xsltExtMarker): Change `extern` to `XSLTPUBVAR` so this
    variable is exported consistently on all platforms.  This
    regressed in commit 44496bf3e.

 libxslt/preproc.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/libxslt/preproc.h b/libxslt/preproc.h
index caf464ad..2a2fc7e4 100644
--- a/libxslt/preproc.h
+++ b/libxslt/preproc.h
@@ -22,7 +22,7 @@ extern "C" {
 /*
  * Interfaces
  */
-extern const xmlChar *xsltExtMarker;
+XSLTPUBVAR const xmlChar *xsltExtMarker;
 
 XSLTPUBFUN xsltElemPreCompPtr XSLTCALL
                xsltDocumentComp        (xsltStylesheetPtr style,