Quash warning emitted by annoylib's debug code if the vertex indices are 64-bit integers

annoylib.h emits debug information if set to verbose, using calls to fprintf or REprintf in RcppAnnoy, with strings that have "%d" variable formatting for the number of nodes. If the AnnoyIndex is instantiated using 64-bit integers for the vertex indices, the compiler will emit a warning if set to `-Wformat` or `-Wall`. This patch (mostly by Claude) wraps the call to REprintf so the warning is no longer emitted.