Makefile: fix 'make dist'

Makefile: fix 'make dist'

Commit

  6c3afe5 ("Validate DNS domain name before powershell invocation")

has introduced a new header file but didn't add it to Makefile's
SOURCES, breaking "make dist".

Change-Id: I93406dd91694974df4ea087b9312ae2fafbc7fe7
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1201
Message-Id: <20250919113207.25440-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg33085.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>