btsnoop: Fix TRUE is undefined for btsnoop socket

Because TRUE / FALSE macro is moved to bt_target.h, there is no more
such definition if including only bt_types.h.

Bug: 163134718
Tag: #refactor
Test: check BTSnoop socket is running when only BT_NET_DEBUG=true
Change-Id: I7ac1d5aeaf3eb4f910cf4498fb7096b6cf4c9fe7