tapaio build fix

With some combinations of compiler flags (no optimization, -Werror), the compiler may complain that the function doesn't have a return statement (even if the function actually just never returns), reported as bug 1128. This adds a dummy return statement.

diff -r 663d59596add tools/blktap/drivers/tapaio.c
@@ -82,6 +82,7 @@ tap_aio_completion_thread(void *arg)
 			}
 		} while (!rc);
 	}
+	return NULL;
 }
 
 void