xmemdup0: Improve GCC 11 allocation-deallocation checking

[Bruno Haible]

2021-08-07  Bruno Haible  <bruno@clisp.org>

        xmemdup0: Improve GCC 11 allocation-deallocation checking.
        * lib/xmemdup0.h: Include <stdlib.h> instead of <stddef.h>.
        (xmemdup0): Declare that deallocation must happen through 'free'.

diff --git a/lib/xmemdup0.h b/lib/xmemdup0.h
index 4f3b581..6e34348 100644
--- a/lib/xmemdup0.h
+++ b/lib/xmemdup0.h
@@ -18,14 +18,16 @@
 #ifndef XMEMDUP_H_
 # define XMEMDUP_H_
 
-# include <stddef.h>
+# include <stdlib.h>
 
 
 # ifdef __cplusplus
 extern "C" {
 # endif
 
-char *xmemdup0 (void const *p, size_t s);
+char *xmemdup0 (void const *p, size_t s)
+  _GL_ATTRIBUTE_MALLOC _GL_ATTRIBUTE_DEALLOC_FREE
+  _GL_ATTRIBUTE_RETURNS_NONNULL;
 
 # ifdef __cplusplus
 }