emailrelay: Remove unnecessary OpenSSL engine include

There's no usage of any ENGINE APIs and trying to include the header
breaks compilation with ENGINE support disabled.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

mail/emailrelay/patches/020-openssl-engine.patch

@@ -0,0 +1,10 @@
+--- a/src/gssl/gssl_openssl.cpp
++++ b/src/gssl/gssl_openssl.cpp
+@@ -32,7 +32,6 @@
+ #include <openssl/ssl.h>
+ #include <openssl/err.h>
+ #include <openssl/rand.h>
+-#include <openssl/engine.h>
+ #include <openssl/conf.h>
+ #include <openssl/evp.h>
+ #include <openssl/hmac.h>