Fix: freeze field type unconditionally

Fix: freeze field type unconditionally

Issue
=====
Field types should always be frozen (in both developer or production
modes) because they are metadata objects. Currently, they are only
frozen in developer mode.

Solution
========
Make the bt_field_type_common_freeze() and bt_field_type_freeze()
functions always called, not through a macro which is only enabled in
developer mode.

Known drawbacks
===============
Very small performance impact because we're trying to freeze a field
type every time bt_field_create() is called, but bt_field_create() is
not called often anyway due to field object pooling. The impact could
become noticeable eventually if we limit the sizes of object pools.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>