get rid of warnings in gcc 4.4.3

This doesn't compile with gcc 4.4.3 on Ubuntu 10.04.4 LTS because it produces the following warning, and warnings are treated as errors.

In file included from rdkafka_performance.c:42:
../rdkafka.h:575: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'rd_kafka_consume_batch'
cc1: warnings being treated as errors
rdkafka_performance.c: In function 'main':
rdkafka_performance.c:722: error: implicit declaration of function 'rd_kafka_consume_batch'

Adding this #include fixes that.