Fix strict-warnings build

crypto/evp/e_aes.c: Types of inp and out parameters of AES_xts_en/decrypt
functions need to be changed from char to unsigned char to avoid build
error due to -Werror=incompatible-pointer-types.

crypto/aes/asm/aes-s390x.pl: Comments need to reflect the above change.

Signed-off-by: Patrick Steuer <psteuer@mail.de>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
CLA: trivial