kconfig: Fix variable name typo %prompts in streamline_config.pl

When I added "use strict;" to streamline_config.pl, I saw the following
warnings:

> Global symbol "%prompt" requires explicit package name at
scripts/kconfig/streamline_config.pl line 183.
> Global symbol "%prompt" requires explicit package name at
scripts/kconfig/streamline_config.pl line 368.

The declaration of %prompt was incorrect, and should have been %prompts.

Cc: Toralf Foerster <toralf.foerster@gmx.de>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Hiromu Yakura <hiromu1996@gmail.com>
LKML-Reference: <1281845597.11566.5.camel@camp10-laptop>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>