tools/squashfskit4: fix build on non-linux systems

The xattr related function calls are linux specific

Signed-off-by: Felix Fietkau <nbd@nbd.name>

tools/squashfskit4/patches/0010-portability.patch

@@ -0,0 +1,37 @@
+--- a/squashfs-tools/xattr.c
++++ b/squashfs-tools/xattr.c
+@@ -113,6 +113,7 @@ static int get_prefix(struct xattr_list
+ 
+ static int read_xattrs_from_system(char *filename, struct xattr_list **xattrs)
+ {
++#if defined(linux)
+ 	ssize_t size, vsize;
+ 	char *xattr_names, *p;
+ 	int i;
+@@ -222,6 +223,10 @@ failed:
+ 	free(xattr_list);
+ 	free(xattr_names);
+ 	return 0;
++#else
++	*xattrs = NULL;
++	return 0;
++#endif
+ }
+ 
+ 
+--- a/squashfs-tools/unsquashfs_xattr.c
++++ b/squashfs-tools/unsquashfs_xattr.c
+@@ -34,6 +34,7 @@ extern int user_xattrs;
+ 
+ void write_xattr(char *pathname, unsigned int xattr)
+ {
++#if defined(linux)
+ 	unsigned int count;
+ 	struct xattr_list *xattr_list;
+ 	int i;
+@@ -136,4 +137,5 @@ void write_xattr(char *pathname, unsigne
+ 	}
+ 
+ 	free_xattr(xattr_list, count);
++#endif
+ }