c-siphash: re-import git-subtree for 'src/c-siphash'

The subtree was moved from "shared/c-siphash" to "src/c-siphash". That confuses
git-subtree. To fix it, reimport the code with

  rm -rf src/c-siphash
  git commit -a -m 'dummy commit'
  git subtree add --prefix src/c-siphash git@github.com:c-util/c-siphash.git eb87a9c4a5b0441ede073597253e1d0b7785e6be --squash

Then rebase the result to drop the bogus dummy commit from the history.

To update the library use:

  git subtree pull --prefix src/c-siphash git@github.com:c-util/c-siphash.git main --squash

src/c-siphash/subprojects/c-stdaux

@@ -1 +1 @@
-Subproject commit 346623b40eb8137cae7568a69ee42253ff098ff7
+Subproject commit c5f166d02ff68af5cdcbad1bdcea2cb134e34ce4