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_client.c b/apps/s_client.c
index 8d78dc7..ba411f2 100644
--- a/apps/s_client.c
+++ b/apps/s_client.c
@@ -925,6 +925,13 @@
             srp_arg.amp = 1;
             meth = TLSv1_client_method();
             break;
+#else
+        case OPT_SRPUSER:
+        case OPT_SRPPASS:
+        case OPT_SRP_STRENGTH:
+        case OPT_SRP_LATEUSER:
+        case OPT_SRP_MOREGROUPS:
+            break;
 #endif
 #ifndef OPENSSL_NO_SSL3
         case OPT_SSL3: