build: depend on tools/zstd for download

Downloading packages from git requires zstd to compress their tarballs.
Make sure that zstd from host tools is compiled when running make
download. Otherwise, either the download would fail because zstd is not
present or a random version from the host would be used leading to hash
mismatches.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>
Link: openwrt/openwrt#21125
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit f3e4620)

Author: Rondom

include/toplevel.mk

@@ -201,7 +201,7 @@ else
   DOWNLOAD_DIRS = package/download
 endif
 
-download: .config FORCE $(if $(wildcard $(STAGING_DIR_HOST)/bin/flock),,tools/flock/compile)
+download: .config FORCE $(if $(wildcard $(STAGING_DIR_HOST)/bin/flock),,tools/flock/compile) $(if $(wildcard $(STAGING_DIR_HOST)/bin/zstd),,tools/zstd/compile)
 	@+$(foreach dir,$(DOWNLOAD_DIRS),$(SUBMAKE) $(dir);)
 
 clean dirclean: .config