Fixes #8 - C linkage error

[bastian-src]

As reported by @NerdyProjects in #8, changes in GLib result in the following build error:

src/falcon-1.3.0/lib/include/falcon/meas/probe_modem.h:24:1: note: ‘extern "C"’ linkage started here
   24 | extern "C" {
      | ^~~~~~~~~~
In file included from /usr/include/glib-2.0/glib/gatomic.h:31,
                 from /usr/include/glib-2.0/glib/gthread.h:32,
                 from /usr/include/glib-2.0/glib/gasyncqueue.h:32,
                 from /usr/include/glib-2.0/glib.h:32,
                 from /usr/include/glib-2.0/gmodule.h:28,
                 from /usr/include/cmnalib/at_sierra_wireless_em7565.h:12,

As discussed here and implemented here the issue can be fixed by moving the include statement out of the extern block.

Tested the build on Ubuntu 22.04 which works fine without further changes.