core: Add function to update osmo_io_ops field for osmo_io_fd

Added, because the field 'io_ops' of 'struct osmo_io_fd' is not a
reference, so subsequent changes to the osmo_io_ops structure that was
used to set it up aren't automatically reflected in the osmo_io_fd
structure that got its copy.

Change-Id: Ie45402ad8e86e3cecf75ad78a512c17e61e68b19
diff --git a/include/osmocom/core/osmo_io.h b/include/osmocom/core/osmo_io.h
index ffc8cfa..c19ca67 100644
--- a/include/osmocom/core/osmo_io.h
+++ b/include/osmocom/core/osmo_io.h
@@ -88,3 +88,5 @@
 
 int osmo_iofd_get_fd(const struct osmo_io_fd *iofd);
 const char *osmo_iofd_get_name(const struct osmo_io_fd *iofd);
+
+void osmo_iofd_set_ioops(struct osmo_io_fd *iofd, const struct osmo_io_ops *ioops);