[LLD] [ELF] Fix building with LLVM_LINK_LLVM_DYLIB since zstd was taken into use

This fixes a regression since fa74144c;
even if we're linking to the dylib (which handles all the dependencies
in LLVMSupport), we're now also directly referencing zstd from
lld/ELF, and thus need to explicitly express our dependency on it.