fix Windows compile - missing xmlWrapOpen.

* xmlIO.c: fix Windows compile - missing xmlWrapOpen.
diff --git a/xmlIO.c b/xmlIO.c
index 848911c..90db848 100644
--- a/xmlIO.c
+++ b/xmlIO.c
@@ -671,7 +671,7 @@
 typedef int (* xmlWrapStatFunc) (const char *f, struct stat *s);
 static xmlWrapStatFunc xmlWrapStat = xmlWrapStatNative;
 typedef FILE* (* xmlWrapOpenFunc)(const char *f,int mode);
-static xmlWrapOpenFunc = xmlWrapOpenNative;
+static xmlWrapOpenFunc xmlWrapOpen = xmlWrapOpenNative;
 
 /**
  * xmlInitPlatformSpecificIo: