Fix #if directives in Producer. (#317)

Some checks that should be #ifdef directives were instead #if directives,
causing headers to not be sent with messages.

Author: florean

confluent_kafka/src/Producer.c

@@ -259,7 +259,7 @@ Producer_producev (Handle *self,
                    const void *value, size_t value_len,
                    const void *key, size_t key_len,
                    void *opaque, int64_t timestamp
-#if RD_KAFKA_V_HEADERS
+#ifdef RD_KAFKA_V_HEADERS
                    ,rd_kafka_headers_t *headers
 #endif
                    ) {
@@ -272,7 +272,7 @@ Producer_producev (Handle *self,
                                  RD_KAFKA_V_VALUE((void *)value,
                                                   (size_t)value_len),
                                  RD_KAFKA_V_TIMESTAMP(timestamp),
-#if RD_KAFKA_V_HEADERS
+#ifdef RD_KAFKA_V_HEADERS
                                  RD_KAFKA_V_HEADERS(headers),
 #endif
                                  RD_KAFKA_V_OPAQUE(opaque),
@@ -386,7 +386,7 @@ static PyObject *Producer_produce (Handle *self, PyObject *args,
                                 value, value_len,
                                 key, key_len,
                                 msgstate, timestamp
-#if RD_KAFKA_V_HEADERS
+#ifdef RD_KAFKA_V_HEADERS
                                 ,rd_headers
 #endif
                                 );