tty: hvc: hvc_vio: Staticify function invoked only by reference

Fixes the following W=1 kernel build warning(s):

 drivers/tty/hvc/hvc_vio.c:181:6: warning: no previous prototype for ‘hvterm_hvsi_hangup’ [-Wmissing-prototypes]
 181 | void hvterm_hvsi_hangup(struct hvc_struct *hp, int data)
 | ^~~~~~~~~~~~~~~~~~

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-33-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/hvc/hvc_vio.c

@@ -178,7 +178,7 @@ static void hvterm_hvsi_close(struct hvc_struct *hp, int data)
 	notifier_del_irq(hp, data);
 }
 
-void hvterm_hvsi_hangup(struct hvc_struct *hp, int data)
+static void hvterm_hvsi_hangup(struct hvc_struct *hp, int data)
 {
 	struct hvterm_priv *pv = hvterm_privs[hp->vtermno];