printk/percpu: Define printk_func when printk is not defined

To avoid include hell, the per_cpu variable printk_func was declared
in percpu.h. But it is only defined if printk is defined.

As users of printk may also use the printk_func variable, it needs to
be defined even if CONFIG_PRINTK is not.

Also add a printk.h include in percpu.h just to be safe.

Link: http://lkml.kernel.org/r/20141121183215.01ba539c@canb.auug.org.au

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/printk/printk.c

@@ -1896,6 +1896,9 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
 			     bool syslog, char *buf, size_t size) { return 0; }
 static size_t cont_print_text(char *text, size_t size) { return 0; }
 
+/* Still needs to be defined for users */
+DEFINE_PER_CPU(printk_func_t, printk_func);
+
 #endif /* CONFIG_PRINTK */
 
 #ifdef CONFIG_EARLY_PRINTK