See upstream PR: https://github.com/kilobyte/compsize/pull/54
--- a/compsize.c
+++ b/compsize.c
@@ -5,12 +5,14 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <dirent.h>
+#include "kerncompat.h"
 #include <btrfs/ioctl.h>
 #include <btrfs/ctree.h>
 #include <stdarg.h>
 #include <stdlib.h>
 #include <sys/ioctl.h>
 #include <inttypes.h>
+#include <errno.h>
 #include <linux/limits.h>
 #include <getopt.h>
 #include <signal.h>
diff --git a/radix-tree.h b/radix-tree.h
index bf96d83..d99ea7e 100644
--- a/radix-tree.h
+++ b/radix-tree.h
@@ -37,11 +37,7 @@
 #ifndef _LINUX_RADIX_TREE_H
 #define _LINUX_RADIX_TREE_H
 
-#if BTRFS_FLAT_INCLUDES
 #include "kerncompat.h"
-#else
-#include <btrfs/kerncompat.h>
-#endif /* BTRFS_FLAT_INCLUDES */
 
 #define RADIX_TREE_MAX_TAGS 2