kernel.h: remove initialization of bool in printk_once

Don't initialize __print_once.  Invert the test to reduce initialized
data.

defconfig before:	$size vmlinux
   text	   data	    bss	    dec	    hex	filename
6976022	 679572	1359668	9015262	 898fde	vmlinux

defconfig after:	$size vmlinux
   text	   data	    bss	    dec	    hex	filename
6976006	 679508	1359700	9015214	 898fae	vmlinux

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>