shared/io-ell: Fix compiler error with GCC 8.0

This fixes the following error:

src/shared/io-ell.c: In function ‘io_set_disconnect_handler’:
src/shared/io-ell.c:117:7: error: cast between incompatible function types from ‘io_callback_func_t’ {aka ‘_Bool (*)(struct io *, void *)’} to ‘void (*)(struct l_io *, void *)’ [-Werror=cast-function-type]
       (l_io_disconnect_cb_t) callback,
       ^
diff --git a/src/shared/io-ell.c b/src/shared/io-ell.c
index 3701f43..6748982 100644
--- a/src/shared/io-ell.c
+++ b/src/shared/io-ell.c
@@ -113,8 +113,7 @@
 	if (!io || !io->l_io)
 		return false;
 
-	return l_io_set_disconnect_handler(io->l_io,
-						(l_io_disconnect_cb_t) callback,
+	return l_io_set_disconnect_handler(io->l_io, (void *) callback,
 							user_data, destroy);
 }