License text fix in some libmm-glib files

[Aleksander Morgado]

Hey all,

A lot of the source files in the libmm-glib library incorrectly had
the GPL text in the beginning of the file instead of the LGPL one.
libmm-glib has always been LGPLv2+ as a whole, and therefore the wrong
text is considered just a bug to fix, not a relicense of any kind.

Fixed here in git main:
https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/689

And here for 1.18:
https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/690

Cheers!

-- 
Aleksander
https://aleksander.es