community/glibmm2.68: upgrade to 2.70.0

• https://ftp.fau.de/gnome/sources/glibmm/2.70/glibmm-2.70.0.news
• https://ftp2.nluug.nl/windowing/gnome/sources/glibmm/2.70/glibmm-2.70.0.changes

>>> Size difference for glibmm2.68: 2924 KiB -> 2940 KiB
>>> No soname differences for glibmm2.68.