headers: add spdx tags to imported files with a known license

headers: add spdx tags to imported files with a known license

I added the header in the cases where the license text is present and it is
easy to find the appropriate SPDX header.

For "public domain" stuff: SDPX treats each "public domain" license as unique [1],
but luckily the one in siphash24.c is one of the identified variants. There are
some other cases which specify "public domain" but there doesn't seem to be a
SPDX identifier.

[1] https://wiki.spdx.org/view/Legal_Team/Decisions/Dealing_with_Public_Domain_within_SPDX_Files

gunicode.[ch] are imported from glib, which is licensed as LGPL2.1+,
see https://gitlab.gnome.org/GNOME/glib/-/blob/master/glib/gunicode.h.