api: fix license headers in MM API

The libmm-glib library, as well as the ModemManager headers installed
as part of the D-Bus service API, have always been LGPLv2+ licensed,
because we want users of the service to rely on them in their own
programs, even if they are proprietary.

Some files had an incorrect GPLv2+ header instead of the LGPLv2+ and
that was confusing. All public headers in the MM API have the correct
license text now.

Signed-off-by: Aleksander Morgado <aleksandermj@chromium.org>