add necessary declarations to build wihtout images

src/helpers.rs

@@ -8,11 +8,14 @@ use std::{
 use crate::{
     common::{BitMatrix, HybridBinarizer},
     multi::{GenericMultipleBarcodeReader, MultipleBarcodeReader},
-    BarcodeFormat, BinaryBitmap, BufferedImageLuminanceSource, DecodeHintType, DecodeHintValue,
+    BarcodeFormat, BinaryBitmap, DecodeHintType, DecodeHintValue,
     DecodingHintDictionary, Exceptions, Luma8LuminanceSource, MultiFormatReader, RXingResult,
     Reader,
 };
 
+#[cfg(feature="image")]
+use crate::BufferedImageLuminanceSource;
+
 #[cfg(feature = "svg_read")]
 pub fn detect_in_svg(
     file_name: &str,
@@ -281,6 +284,8 @@ pub fn save_file(file_name: &str, bit_matrix: &BitMatrix) -> Result<(), Exceptio
         String::default()
     };
 
+    let _ = &ext;
+
     #[cfg(feature = "svg_write")]
     if ext == "svg" {
         return save_svg(file_name, bit_matrix);