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

This fixes a regression since fa74144c64dff6b145b0b3fa9397f913ddaa87bf;
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.

lld/ELF/CMakeLists.txt

@@ -6,6 +6,18 @@ if(LLVM_ENABLE_ZLIB)
   set(imported_libs ZLIB::ZLIB)
 endif()
 
+if(LLVM_ENABLE_ZSTD)
+  if(TARGET zstd::libzstd_shared AND NOT LLVM_USE_STATIC_ZSTD)
+    set(zstd_target zstd::libzstd_shared)
+  else()
+    set(zstd_target zstd::libzstd_static)
+  endif()
+endif()
+
+if(LLVM_ENABLE_ZSTD)
+  list(APPEND imported_libs ${zstd_target})
+endif()
+
 add_lld_library(lldELF
   AArch64ErrataFix.cpp
   Arch/AArch64.cpp