crypto/evp/evp_key.c: #define BUFSIZ if <stdio.h> doesn't #define it

CLA: trivial

Fixes #8904

Commit 48feaceb ("Remove the possibility to disable the UI module
entirely", 2017-07-03) made the BUFSIZ references in "evp_key.c"
unconditional, by deleting the preprocessing directive "#ifndef
OPENSSL_NO_UI". This breaks the build when compiling OpenSSL for edk2
(OPENSSL_SYS_UEFI), because edk2's <stdio.h> doesn't #define BUFSIZ.

Provide a fallback definition, like we do in "crypto/ui/ui_util.c" (from
commit 984d6c60

, "Fix no-stdio build", 2015-09-29).

Signed-off-by: Laszlo Ersek <lersek@redhat.com>

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8922)