changed code to avoid a compiler warning

dlib/opencv/cv_image.h

@@ -84,7 +84,11 @@ namespace dlib
 
     private:
 
-        inline void check_image_type (const IplImage* img) const
+        inline void check_image_type (const IplImage* 
+#ifdef ENABLE_ASSERTS
+                                      img // the #ifdef is here to avoid an unused warning argument from the compiler
+#endif
+                                      ) const
         {
             DLIB_ASSERT( img->dataOrder == 0, "Only interleaved color channels are supported with cv_image"); 
             DLIB_ASSERT((img->depth&0xFF)/8*img->nChannels == sizeof(pixel_type),