Remove implicit int declaration from a test

This didn't seem to be the point of the issue but caused the test to
fail.

Error message:
Common.TranslateAssertError@tests/unit/issues/Issue8.d(14):
Cannot compile source code. Errors:
----------------
 /tmp/dstepZRUFDHDGRGxkGPVu.h:22:1: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]

Author: Inkrementator

tests/unit/issues/Issue8.d

@@ -33,7 +33,7 @@ typedef struct rd_kafka_metadata {
         char       *orig_broker_name; /* Name of originating broker */
 } rd_kafka_metadata_t;
 
-rd_kafka_metadata (rd_kafka_t *rk, int all_topics,
+int rd_kafka_metadata (rd_kafka_t *rk, int all_topics,
                    rd_kafka_topic_t *only_rkt,
                    const struct rd_kafka_metadata **metadatap,
                    int timeout_ms);