libfdt: Replace deprecated 0-length arrays with proper flexible arrays

Replace the 0-length arrays in structures with proper flexible
arrays. This will avoid warnings when building under GCC 13 with
-fstrict-flex-arrays, which the Linux kernel will be doing soon:

In file included from ../lib/fdt_ro.c:2:
../lib/../scripts/dtc/libfdt/fdt_ro.c: In function 'fdt_get_name':
../lib/../scripts/dtc/libfdt/fdt_ro.c:319:24: warning: 'strrchr' reading 1 or more bytes from a region of size 0 [-Wstringop-overread]
  319 |                 leaf = strrchr(nameptr, '/');
      |                        ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt.h

@@ -35,14 +35,14 @@ struct fdt_reserve_entry {
 
 struct fdt_node_header {
 	fdt32_t tag;
-	char name[0];
+	char name[];
 };
 
 struct fdt_property {
 	fdt32_t tag;
 	fdt32_t len;
 	fdt32_t nameoff;
-	char data[0];
+	char data[];
 };
 
 #endif /* !__ASSEMBLY */