Use rb_pid_t because pid_t isn't defined on some Windows systems.

ext/sqlite3/database.h

@@ -8,7 +8,7 @@ struct _sqlite3Ruby {
     VALUE busy_handler;
     int stmt_timeout;
     struct timespec stmt_deadline;
-    pid_t owner;
+    rb_pid_t owner;
 };
 
 typedef struct _sqlite3Ruby sqlite3Ruby;