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>
diff --git a/apps/s_server.c b/apps/s_server.c
index c54caa6..2ef902a 100644
--- a/apps/s_server.c
+++ b/apps/s_server.c
@@ -1317,6 +1317,10 @@
             srpuserseed = opt_arg();
             meth = TLSv1_server_method();
             break;
+#else
+        case OPT_SRPVFILE:
+        case OPT_SRPUSERSEED:
+            break;
 #endif
         case OPT_REV:
             rev = 1;