Add separate SSL_COOKIE_C define
diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index abf9e15..86cdc12 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -33,7 +33,7 @@
 #include POLARSSL_CONFIG_FILE
 #endif
 
-#if defined(POLARSSL_SSL_DTLS_HELLO_VERIFY)
+#if defined(POLARSSL_SSL_COOKIE_C)
 
 #include "polarssl/ssl_cookie.h"