io_uring: FileHandles should convert to int

The are always used for FD arguments, which are generally of type int.

Change-Id: I6bdd3b27febaa42579546669b7368e297b87bf40
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/stdplus/io_uring.hpp b/src/stdplus/io_uring.hpp
index d5f33e8..71ce17a 100644
--- a/src/stdplus/io_uring.hpp
+++ b/src/stdplus/io_uring.hpp
@@ -33,7 +33,7 @@
     class FileHandle
     {
       public:
-        inline operator unsigned() const
+        inline operator int() const
         {
             return *slot;
         }