Dynamic debug: fix pr_fmt() build error

When CONFIG_DYNAMIC_DEBUG is enabled, allow callers of pr_debug()
to provide their own definition of pr_fmt() even if that definition
uses tricks like

#define pr_fmt(fmt) "%s:" fmt, __func__

Signed-off-by: Greg Banks <gnb@sgi.com>
Cc: Jason Baron <jbaron@redhat.com>
Acked-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>