BUILD: sample: include openssl-compat

It's needed for EVP_*.
diff --git a/src/sample.c b/src/sample.c
index 00b8202..ad15d04 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -33,6 +33,7 @@
 #include <haproxy/istbuf.h>
 #include <haproxy/mqtt.h>
 #include <haproxy/net_helper.h>
+#include <haproxy/openssl-compat.h>
 #include <haproxy/protobuf.h>
 #include <haproxy/proxy.h>
 #include <haproxy/regex.h>