Fix strict-warnings build

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: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
CLA: trivial