Changes

sigtran: xua_msg.h,msg_types.h: Make headers private

Those headers are only used internally in libosmo-sigtran, since those
objects have no real use outside of it. Mark the headers as no-install,
which implicitly makes them private.

This decreases the amount of public APIs the libosmo-sigtran has.

Change-Id: Icd23bf9c9364c33ea2ba709e3ef4439813a168f4