Add missing aes.c placeholder which includes lib/aes.c . Without this
one, tools/env/ will fail to build.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Alexey Brodkin <abrodkin@synopsys.com>
Tested-by: Heiko Schocher <hs@denx.de>