Normalize C source files to end with exactly one newline

Some editors automatically remove trailing blank lines, or automatically add a trailing newline to avoid having a trailing non-blank line that is not terminated by a newline. To avoid unrelated whitespace changes when users of such editors contribute to dbus, let's pre-emptively normalize all files.

Unlike more intrusive whitespace normalization like removing trailing whitespace from each line, this seems unlikely to cause significant issues with cherry-picking changes to stable branches.

Implemented by:

find . -name '*.[ch]' -print0 | \
xargs -0 perl -0777 -p -i -e 's/\n+\z//g; s/\z/\n/g'