libsql-sqlite3: Skip SQLITE_USER_AUTHENTICATION #warning on MSVC

The SQLITE_USER_AUTHENTICATION deprecation #warning (added upstream in
SQLite 3.46.0) is a hard error under MSVC's default traditional C
preprocessor (error C1021: invalid preprocessor command 'warning'),
breaking Windows builds. SQLite3MultipleCiphers force-enables
SQLITE_USER_AUTHENTICATION by default, so the directive is active even
though build.rs passes -DSQLITE_USER_AUTHENTICATION=OFF.

Guard the directive with !defined(_MSC_VER) so the deprecation notice is
kept for GCC/Clang and skipped on MSVC. This is purely a compile-time
diagnostic; the extension's runtime behavior is unchanged on all
platforms.

Upstream removed this extension entirely in SQLite 3.48.0 (commit
bc4df6079c), so this local patch becomes moot once the bundled SQLite is
updated past 3.47.0. The patch is applied in libsql-sqlite3/src and
mirrored into both regenerated bundled amalgamations.