readline: Improve GCC 11 allocation-deallocation checking

[Bruno Haible]

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

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

diff --git a/lib/readline.h b/lib/readline.h
index 57acdf7..e4b6f9b 100644
--- a/lib/readline.h
+++ b/lib/readline.h
@@ -24,11 +24,13 @@
 # include <stdio.h>
 # include <readline/readline.h>
 #else
+# include <stdlib.h>
 /* Prints a prompt PROMPT and then reads and returns a single line of
    text from the user.  If PROMPT is NULL or the empty string, no
    prompt is displayed.  The returned line is allocated with malloc;
    the caller should free the line when it has finished with it. */
-extern char *readline (const char *prompt);
+extern char *readline (const char *prompt)
+  _GL_ATTRIBUTE_MALLOC _GL_ATTRIBUTE_DEALLOC_FREE;
 #endif
 
 #endif /* GL_READLINE_H */