serial: Fix warnings caused by missing 'static' attribute

serial: Fix warnings caused by missing 'static' attribute

Warnings from the Sparse static analysis tool:

hw/char/serial.c:630:26: warning: symbol
 'vmstate_serial_thr_ipending' was not declared. Should it be static?
hw/char/serial.c:646:26: warning: symbol
 'vmstate_serial_tsr' was not declared. Should it be static?
hw/char/serial.c:665:26: warning: symbol
 'vmstate_serial_recv_fifo' was not declared. Should it be static?
hw/char/serial.c:681:26: warning: symbol
 'vmstate_serial_xmit_fifo' was not declared. Should it be static?
hw/char/serial.c:697:26: warning: symbol
 'vmstate_serial_fifo_timeout_timer' was not declared. Should it be static?
hw/char/serial.c:713:26: warning: symbol
 'vmstate_serial_timeout_ipending' was not declared. Should it be static?
hw/char/serial.c:729:26: warning: symbol
 'vmstate_serial_poll' was not declared. Should it be static?

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>