libc: Move gmtime_r into common

gmtime_r() has been in the minimal libc for years, however it was not
added to expcetions due to an overlook. In order to do this however, it
has to be moved first to the common libc area, so that it's available
to any libc that may not implement it.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>