Fix the compile error once enabled Werror

Fix the compile error once enabled Werror

On 32 bit operating system,size_t is defined as unsigned int,
this is the return type of strlen(), but it isn't aligned with the %ld,
when compiling, warning will be reported.
Change the type to %zu to avoid the warning.

Change-Id: I2943d0dfba88ef42892f14230242008473d6263b

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11325)