Hide [[gnu::malloc(deallocator)]] in a macro

Clang doesn't implement this attribute and reports an error.  Work
around it by hiding it in a macro that will be empty in clang.

Reported-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/prototypes.h

@@ -47,7 +47,7 @@ extern int expire (const struct passwd *, /*@null@*/const struct spwd *);
 
 /* agetpass.c */
 extern void erase_pass(char *pass);
-[[gnu::malloc(erase_pass)]]
+ATTR_MALLOC(erase_pass)
 extern char *agetpass(const char *prompt);
 
 /* isexpired.c */