security/heimdal: Fix build

Three problems were discovered when building under poudriere or in
a clean jail.

1. Python is now a prerequisite.

2. liblockfile is now needed.

3. clang-format is needed for asn1_compile. Unfortunately the base llvm does not install clang-format so we need install $LLVM_DEFAULT to get this file.

PR: 267814
Reported by: many
Fixes: 83f79ba0e0ca

(cherry picked from commit a5523d807d01b1ed31614f346db2b348d7046420)