include/linux/printk.h: include stdarg.h

printk.h uses va_list but doesn't include stdarg.h.  Hence printk.h is
unusable unless its includer has already included kernel.h (which includes
stdarg.h).

Remove the dependency by including stdarg.h in printk.h
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/printk.h

 #ifndef __KERNEL_PRINTK__
 #define __KERNEL_PRINTK__
 
+#include <stdarg.h>
 #include <linux/init.h>
 #include <linux/kern_levels.h>