Add empty stub for virThreadCancel on Win32

Win32 does not like undefined symbols, so define an
empty virThreadCancel impl.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/util/virthreadwin32.c

@@ -357,6 +357,8 @@ void virThreadJoin(virThreadPtr thread)
     }
 }
 
+void virThreadCancel(virThreadPtr thread ATTRIBUTE_UNUSED)
+{}
 
 int virThreadLocalInit(virThreadLocalPtr l,
                        virThreadLocalCleanup c)