Missing include for `dlib::loss_multiclass_log_per_pixel_` (#2432)

* Missing include for `dlib::loss_multiclass_log_per_pixel_::label_to_ignore`

I was trying to compile the examples and encountered this issue after moving `rgb_label_image_to_index_label_image` to cpp file. Headers should include all symbols they mention.

* Update pascal_voc_2012.h

Should use the official entrypoint for including dnn stuff.
Co-authored-by: Davis E. King <davis685@gmail.com>

examples/pascal_voc_2012.h

@@ -7,6 +7,7 @@
 #define PASCAL_VOC_2012_H_
 
 #include <dlib/pixel.h>
+#include <dlib/dnn.h>
 
 // ----------------------------------------------------------------------------------------