Remove #error from include files.

Don't have #error statements in header files, but instead wrap
the contents of that file in #ifndef OPENSSL_NO_xxx
This means it is now always safe to include the header file.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/ssl_sess.c

@@ -137,9 +137,7 @@
 #include <stdio.h>
 #include <openssl/lhash.h>
 #include <openssl/rand.h>
-#ifndef OPENSSL_NO_ENGINE
-# include <openssl/engine.h>
-#endif
+#include <openssl/engine.h>
 #include "ssl_locl.h"
 
 static void SSL_SESSION_list_remove(SSL_CTX *ctx, SSL_SESSION *s);