lib/plist.c: replace pr_debug with printk in plist_test()

Replace pr_debug() in lib/plist.c test function plist_test() with
printk(KERN_DEBUG ...).

Without DEBUG defined, pr_debug() is complied out, but the entire
plist_test() function is already inside CONFIG_DEBUG_PI_LIST, so printk
should just be used directly.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>