discover-image: export search paths array

This way we can use it to validate image paths later.

src/shared/discover-image.c

@@ -48,7 +48,7 @@
 #include "vpick.h"
 #include "xattr-util.h"
 
-static const char* const image_search_path[_IMAGE_CLASS_MAX] = {
+const char* const image_search_path[_IMAGE_CLASS_MAX] = {
         [IMAGE_MACHINE] =   "/etc/machines\0"              /* only place symlinks here */
                             "/run/machines\0"              /* and here too */
                             "/var/lib/machines\0"          /* the main place for images */

src/shared/discover-image.h

@@ -122,3 +122,5 @@ int image_to_json(const struct Image *i, JsonVariant **ret);
 const char *image_root_to_string(ImageClass c) _const_;
 
 extern const struct hash_ops image_hash_ops;
+
+extern const char* const image_search_path[_IMAGE_CLASS_MAX];