Clean zstd files

Summary:
zstd files are downloaded and used as part of JNI build, but are left behind even after doing a `make clean`. This PR updates the `clean` target to remove these zstd files as well.
Closes https://github.com/facebook/rocksdb/pull/2365

Differential Revision: D5123537

Pulled By: sagar0

fbshipit-source-id: a8f355da5ba961aa89d5852e35751ffc35de03ea

Makefile

@@ -902,7 +902,7 @@ clean:
 	rm -rf $(CLEAN_FILES) ios-x86 ios-arm scan_build_report
 	find . -name "*.[oda]" -exec rm -f {} \;
 	find . -type f -regex ".*\.\(\(gcda\)\|\(gcno\)\)" -exec rm {} \;
-	rm -rf bzip2* snappy* zlib* lz4*
+	rm -rf bzip2* snappy* zlib* lz4* zstd*
 	cd java; $(MAKE) clean
 
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