Put canonicaliseTimestampAndPermissions in its own header/file

It is not inherently tied to `LocalStore`, it could probably even go in
`libnixutil`. Functions not attached to `LocalStore` should not be
declared in `local-store.hh`.

I am moving it to facilitate experimenting for #9344. If
canonicalisation should be done client-side in client-side builds, there
wouldn't be a `LocalStore` at all so having to include that header to
get this freestanding function is cumbersome and wrong.

Perhaps canonicalisation should still be done server-side for security
reasons --- I don't mean to make that judgement call now --- but even if
so, this freestanding function still isn't connected to `LocalStore` so
while less urgent it is still better to move out of this header.

src/libstore/build/local-derivation-goal.cc

@@ -19,6 +19,7 @@
 #include "namespaces.hh"
 #include "child.hh"
 #include "unix-domain-socket.hh"
+#include "posix-fs-canonicalise.hh"
 
 #include <regex>
 #include <queue>