kmsg_dump: fix build for CONFIG_PRINTK=n

kmsg_dump: fix build for CONFIG_PRINTK=n

kmsg_dump() fails to build when CONFIG_PRINTK=n; provide stubs
for the kmsg_dump*() functions when CONFIG_PRINTK=n.

kernel/printk.c: In function 'kmsg_dump':
kernel/printk.c:1501: error: 'log_buf_len' undeclared (first use in this function)
kernel/printk.c:1502: error: 'logged_chars' undeclared (first use in this function)
kernel/printk.c:1506: error: 'log_buf' undeclared (first use in this function)

Signed-off-by: Randy Dunlap <[email protected]>
Acked-by: Simon Kagstrom <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>