subsys/fs/fuse: Fix adding missing stddef header

Instead of relaying on stddef.h being included by other headers
let's include it explicitly, following a report of it being
missing for Ubuntu 22.04 (glibc 2.35)

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

subsys/fs/fuse_fs_access_bottom.c

@@ -10,6 +10,7 @@
 #undef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 700
 
+#include <stddef.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>