fix rd_kafka_event_type method signature (#768)

Author: mhowlett

src/Confluent.Kafka/Impl/NativeMethods/NativeMethods.cs

@@ -688,7 +688,7 @@ internal static extern void rd_kafka_DescribeConfigs (
         internal static extern void rd_kafka_event_destroy(IntPtr rkev);
 
         [DllImport(DllName, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern int rd_kafka_event_type(IntPtr rkev);
+        internal static extern Librdkafka.EventType rd_kafka_event_type(IntPtr rkev);
 
         [DllImport(DllName, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr rd_kafka_event_opaque(IntPtr rkev);

src/Confluent.Kafka/Impl/NativeMethods/NativeMethods_Centos7.cs

@@ -688,7 +688,7 @@ internal static extern void rd_kafka_DescribeConfigs (
         internal static extern void rd_kafka_event_destroy(IntPtr rkev);
 
         [DllImport(DllName, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern int rd_kafka_event_type(IntPtr rkev);
+        internal static extern Librdkafka.EventType rd_kafka_event_type(IntPtr rkev);
 
         [DllImport(DllName, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr rd_kafka_event_opaque(IntPtr rkev);

src/Confluent.Kafka/Impl/NativeMethods/NativeMethods_Debian9.cs

@@ -688,7 +688,7 @@ internal static extern void rd_kafka_DescribeConfigs (
         internal static extern void rd_kafka_event_destroy(IntPtr rkev);
 
         [DllImport(DllName, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern int rd_kafka_event_type(IntPtr rkev);
+        internal static extern Librdkafka.EventType rd_kafka_event_type(IntPtr rkev);
 
         [DllImport(DllName, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr rd_kafka_event_opaque(IntPtr rkev);