[CMake] Add missing build dependency for builtin zstd

builtins/zstd defined the imported ZSTD::ZSTD target but never tied it to
the BUILTIN_ZSTD ExternalProject that actually produces libzstd.a. As a
result a parallel or targeted build (e.g. building only the Core target)
could try to link the archive before it exists. Add the dependency edge,
consistent with what builtins/zlib already does for BUILTIN_ZLIB.

🤖 Done with the help of [Claude Code](https://claude.com/claude-code) (Claude Opus 4.8)

Author: guitargeek

builtins/zstd/CMakeLists.txt

@@ -62,6 +62,10 @@ set_target_properties(ZSTD::ZSTD PROPERTIES
     IMPORTED_LOCATION ${ROOT_ZSTD_LIBRARY}
     INTERFACE_INCLUDE_DIRECTORIES ${ZSTD_INCLUDE_DIR}
 )
+# Make consumers of ZSTD::ZSTD wait for the ExternalProject that actually builds
+# the library, otherwise a parallel or targeted build can try to link the not-yet
+# existing libzstd archive (consistent with builtins/zlib's add_dependencies).
+add_dependencies(ZSTD::ZSTD BUILTIN_ZSTD)
 
 # Set the canonical output of find_package according to
 # https://cmake.org/cmake/help/latest/manual/cmake-developer.7.html#standard-variable-names