tests: Fix link errors for t-thread-local.

On platforms where pthread_create is not in libc t-thread-local fails to
link. Issue found on NetBSD 10.0.

• tests/Makefile.am (t_thread_local_LDADD): Add $(standard_ldadd),

$(GPG_ERROR_MT_LIBS), and @LDADD_FOR_TESTS_KLUDGE@.
(t_thread_local_CFLAGS): Add $(GPG_ERROR_MT_CFLAGS).

• GnuPG-bug-id: T7634
• Signed-off-by: Collin Funk <collin.funk1@gmail.com>