Fix compilation without OpenEXR

Commit e38d2c8 "add more magic bytes, use only appropriate loader when magic matches" introduced surrogate loader functions for file types whose libraries are not available. It was missing a surrogate for OpenEXR resulting in compilation fail if the library was not enabled. This commit adds the missing surrogate.

Author: tomaszg7

src/imageio/imageio.c

@@ -179,6 +179,10 @@ static dt_imageio_retval_t _unsupported_type(dt_image_t *img,
 #define dt_imageio_open_im _unsupported_type
 #endif
 
+#ifndef HAVE_OPENEXR
+#define dt_imageio_open_exr _unsupported_type
+#endif
+
 typedef struct {
   dt_filetype_t filetype;
   gboolean      hdr;