Add missing include file

To fix build after Tarantool update.

Author: olegrok

kafka/consumer_msg.c

@@ -1,4 +1,5 @@
 #include <stdlib.h>
+#include <string.h>
 
 #include <tarantool/module.h>
 

kafka/producer.c

@@ -1,6 +1,7 @@
 #include <unistd.h>
 #include <stdlib.h>
 #include <errno.h>
+#include <string.h>
 
 #include <librdkafka/rdkafka.h>
 #include <tarantool/module.h>