[WATCHDOG] mpc8xxx_wdt: fix modular build

Fix the following build error when mpc8xxx_wdt is selected to build as a
module:

drivers/watchdog/mpc8xxx_wdt.c:304: error: redefinition of '__inittest'
drivers/watchdog/mpc8xxx_wdt.c:298: error: previous definition of '__inittest' was here
drivers/watchdog/mpc8xxx_wdt.c:304: error: redefinition of 'init_module'
drivers/watchdog/mpc8xxx_wdt.c:298: error: previous definition of 'init_module' was here

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>