Add function gpgrt_annotate_leaked_object.

* src/gpg-error.h.in: add gpgrt_annotate_leaked_object to support
  marking memory as non-leaked for Clang and GCC.

This annotation can be used to mark objects as explicitly leaked such
that it can be ignored in tools like LeakSanitizer.

The GPGRT_HAVE_LEAK_SANITIZER macro is explicitly not undefined to
support -fsanitize=leak, a user or configure script could then decide to
add this macro when just -fsanitize=leak is given.

Additional changes by -wk:

• But GNUC guard around the entire GPGRT_HAVE_LEAK_SANITIZER detection.
• Add NEWS entry.

• Signed-off-by: Werner Koch <wk@gnupg.org>