watchdog: Missing Kconfig dependency for CONFIG_WATCHDOG_PRETIMEOUT_GOV and CONFIG_WATCHDOG_CORE

["Kuo, Hsuan-Chi" <hckuo2@illinois.edu>]

Hi,

I compiled my kernel with CONFIG_WATCHDOG_PRETIMEOUT_GOV=y and
CONFIG_WATCHDOG_CORE=n and encountered compilation errors. I later
found in the Makefile that it's because CONFIG_WATCHDOG_CORE is not
enabled hence watchdog.o is not compiled, however, the Kconfig misses
this dependency. I sent this email to confirm if this is a bug and
hopfully someone can fix it.

Snippet for the Makefile
```
# Makefile for the WatchDog device drivers.
#

# The WatchDog Timer Driver Core.
obj-$(CONFIG_WATCHDOG_CORE)    += watchdog.o

watchdog-objs    += watchdog_core.o watchdog_dev.o

watchdog-$(CONFIG_WATCHDOG_PRETIMEOUT_GOV)    += watchdog_pretimeout.o

obj-$(CONFIG_WATCHDOG_PRETIMEOUT_GOV_NOOP)    += pretimeout_noop.o
obj-$(CONFIG_WATCHDOG_PRETIMEOUT_GOV_PANIC)    += pretimeout_panic.o
```

Snippet for the Kconfig
```
config WATCHDOG_PRETIMEOUT_GOV
	bool "Enable watchdog pretimeout governors"
	help
	  The option allows to select watchdog pretimeout governors.
```

Compilation errors
```
ld: drivers/watchdog/pretimeout_noop.o: in function
`watchdog_gov_noop_unregister':
pretimeout_noop.c:(.exit.text+0x8): undefined reference to
`watchdog_unregister_governor'
ld: drivers/watchdog/pretimeout_noop.o: in function
`watchdog_gov_noop_register':
pretimeout_noop.c:(.init.text+0xd): undefined reference to
`watchdog_register_governor'
ld: drivers/watchdog/pretimeout_panic.o: in function
`watchdog_gov_panic_unregister':
pretimeout_panic.c:(.exit.text+0x8): undefined reference to
`watchdog_unregister_governor'
ld: drivers/watchdog/pretimeout_panic.o: in function
`watchdog_gov_panic_register':
pretimeout_panic.c:(.init.text+0xd): undefined reference to
`watchdog_register_governor'
```

Thank you!