[llvm] [test] Add missing canonicalization of LLVM_ENABLE_ZSTD

Add LLVM_ENABLE_ZSTD to llvm_canonicalize_cmake_booleans().  This is
needed to ensure that the substitutions in lit.site.cfg.py resolve
to correct Python booleans.

Differential Revision: https://reviews.llvm.org/D135357

(cherry picked from commit bc4bcbcf)