CVE-2022-3437 third_party/heimdal: Remove __func__ compatibility workaround

CVE-2022-3437 third_party/heimdal: Remove __func__ compatibility workaround

As described by the C standard, __func__ is a variable, not a macro.
Hence this #ifndef check does not work as intended, and only serves to
unconditionally disable __func__. A nonoperating __func__ prevents
cmocka operating correctly, so remove this definition.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15134

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>