Use #error in openssl/srp.h

Follow the same convention the other OPENSSL_NO_xxx header files
do, and use #error instead of making the header file be a no-op.

Reviewed-by: Matt Caswell <matt@openssl.org>

ssl/tls_srp.c

@@ -60,11 +60,11 @@
 
 #include <openssl/crypto.h>
 #include <openssl/rand.h>
-#include <openssl/srp.h>
 #include <openssl/err.h>
 #include "ssl_locl.h"
 
 #ifndef OPENSSL_NO_SRP
+#include <openssl/srp.h>
 
 int SSL_CTX_SRP_CTX_free(struct ssl_ctx_st *ctx)
 {