oslib-win32: do not rely on macro to get redefined function name

On Windows with glib <2.50, g_poll is redefined to use the variant
defined in util/oslib-win32.c.  Use the same name in the declaration
and definition for ease of grepping.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/oslib-win32.c

@@ -472,7 +472,7 @@ static int poll_rest(gboolean poll_msgs, HANDLE *handles, gint nhandles,
     return 0;
 }
 
-gint g_poll(GPollFD *fds, guint nfds, gint timeout)
+gint g_poll_fixed(GPollFD *fds, guint nfds, gint timeout)
 {
     HANDLE handles[MAXIMUM_WAIT_OBJECTS];
     gboolean poll_msgs = FALSE;