printk: add stub for prepend_timestamp()

Add a stub for prepend_timestamp() when CONFIG_PRINTK is not
enabled.  Fixes this build error:

kernel/printk.c:1770:3: error: implicit declaration of function 'prepend_timestamp'

Cc: Kay Sievers <kay@vrfy.org>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/printk.c

@@ -1439,6 +1439,10 @@ static struct log *log_from_idx(u32 idx) { return NULL; }
 static u32 log_next(u32 idx) { return 0; }
 static char *log_text(const struct log *msg) { return NULL; }
 static void call_console_drivers(int level, const char *text, size_t len) {}
+static size_t prepend_timestamp(unsigned long long t, char *buf)
+{
+	return 0;
+}
 
 #endif /* CONFIG_PRINTK */