Fix warnings when building with Visual Studio.

src/channel.c

@@ -400,7 +400,7 @@ static int check_body(belle_sip_channel_t *obj){
 }
 
 static int acquire_body_simple(belle_sip_channel_t *obj, int end_of_stream){
-	int content_length=obj->input_stream.content_length;
+	size_t content_length=obj->input_stream.content_length;
 	size_t to_read=obj->input_stream.write_ptr-obj->input_stream.read_ptr;
 	belle_sip_message_t *msg=obj->input_stream.msg;
 	belle_sip_body_handler_t *bh=belle_sip_message_get_body_handler(msg);

src/channel.h

@@ -76,7 +76,7 @@ typedef struct belle_sip_channel_input_stream{
 	char* read_ptr;
 	char* write_ptr;
 	belle_sip_message_t *msg;
-	int content_length;
+	size_t content_length;
 	int chuncked_mode;
 	int chunk_size;
 	int chunk_read_size;