hw/xwin: Fix redundant declaration in winclipboardinit.c

In file included from winclipboardinit.c:35:0:
winclipboard.h:81:13: error: redundant redeclaration of ‘ErrorF’
../../include/os.h:558:13: note: previous declaration of ‘ErrorF’ was here

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

hw/xwin/winclipboard.h

@@ -79,7 +79,6 @@
  */
 
 extern char *display;
-extern void ErrorF(const char * /*f */ , ...);
 extern void winDebug(const char *format, ...);
 extern void winErrorFVerb(int verb, const char *format, ...);