cdefs: allow __attribute__ on tcc

cdefs: allow __attribute__ on tcc

According to the tcc (tiny C compiler) Changelog, tcc supports
__attribute__ since 0.9.3.  Looking at history of tcc at
<https://repo.or.cz/tinycc.git>, __attribute__ support was added
in commit 14658993425878be300aae2e879560698e0c6c4c on 2002-01-03,
which also looks like the release of 0.9.3.  While I'm unable to
find release tags for tcc before 0.9.18 (2003-04-14), the next
release (0.9.28) will include __attribute__((cleanup(func)) which
I rely on.

Reviewed-by: Collin Funk <collin.funk1@gmail.com>