Moved declaration of format-specific image Load functions to dedicated header

The functions are declared in different files but they were called
in another file which re-declared the prototypes. This is dangerous
as it can lead to misaligned prototypes when functions changes.

They are now grouped in the library internal header 'imgformat.h'

wrlib/nxpm.c

@@ -29,6 +29,7 @@
 #include <errno.h>
 
 #include "wraster.h"
+#include "imgformat.h"
 
 /*
  * Restricted support for XPM images.