[WATCHDOG] fix watchdog/txx9wdt.c compilation

This patch fixes the following compile error caused by
commit 8dc244f7
([WATCHDOG 48/57] txx9: Fix locking, switch to unlocked_ioctl):

<--  snip  -->

...
  CC      drivers/watchdog/txx9wdt.o
txx9wdt.c:48: warning: type defaults to 'int' in declaration of 
txx9wdt.c:48: warning: parameter names (without types) in function 
txx9wdt.c: In function 'txx9wdt_ping':
txx9wdt.c:52: error: 'txx9_lock' undeclared (first use in this function)
txx9wdt.c:52: error: (Each undeclared identifier is reported only once
txx9wdt.c:52: error: for each function it appears in.)
txx9wdt.c: In function 'txx9wdt_start':
txx9wdt.c:59: error: 'txx9_lock' undeclared (first use in this function)
txx9wdt.c: In function 'txx9wdt_stop':
txx9wdt.c:71: error: 'txx9_lock' undeclared (first use in this function)
make[3]: *** [drivers/watchdog/txx9wdt.o] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>