The BTS constructor uses functions of libosmocore that could in turn
want to log something.  This requires the logging to be initialized
before.

The only way to achieve this is to add an __attribute__((constructor))
function *before* the BTS constructor is being run.

This solution might not be elegant, but  I guess it's the only way to
initialize a C library before calling C++ constructors of global static
instance of a class.

In case anyone comes up with a better / cleaner approach, we can always
change later.

This change requires libosmocore >= 0.10.1, as only that permits
multiple calls to osmo_init_logging() which may now occur.

Change-Id: I28dc4f0db229518348c92413959fed5ae85d753d