Fix duplicate local variable declaration in busy handler

C89-style: cancelled was declared twice in the same function scope after
the data-race fix split the lock sections. Remove the second 'int' to
reuse the variable declared at the top of the function.

Author: mjc

c_src/sqlite3_nif.c

@@ -474,7 +474,7 @@ exqlite_busy_handler(void* arg, int count)
 
     // Wait on the condvar — can be woken early by cancel()
     tw_lock(&conn->cancel_tw);
-    int cancelled = conn->cancelled;
+    cancelled = conn->cancelled;
     if (!cancelled) {
         tw_wait_ms(&conn->cancel_tw, sleep_ms);
         cancelled = conn->cancelled; // snapshot again after waking