add necessary declarations to build wihtout images

src/multi/mod.rs

@@ -9,4 +9,5 @@ mod generic_multiple_barcode_reader;
 pub use generic_multiple_barcode_reader::*;
 
 #[cfg(test)]
+#[cfg(feature="image")]
 mod multi_test_case;

src/multi/qrcode/qr_code_multi_reader.rs

@@ -205,6 +205,7 @@ fn compareRXingResult(a: &RXingResult, b: &RXingResult) -> Ordering {
 }
 
 #[cfg(test)]
+#[cfg(feature="image")]
 mod multi_qr_code_test_case {
     /*
      * Copyright 2016 ZXing authors