copyright.py: Handle files in gdb/gnulib owned by GDB.

The script was excluding all of gdb/gnulib but this is no longer
correct, ever since we moved the imported files to gdb/gnulib/import.
As a result, a number of files (Makefile, etc, including this script
itself) did not have their copyright header updated. This fixes
the problem.

gdb/ChangeLog:

        * copyright.py (EXCLUDE_LIST): Replace "gdb/gnulib" by
        "gdb/gnulib/import".
Update dates in relevant gdb/gnulib/ copyright headers

This patch is the result of re-running the copyright.py script
in GDB, after we modified it to stop ignoring some files in
gdb/gnulib that should have been updated earlier this year.

gdb/ChangeLog:

        * gdb/gnulib/Makefile.in: Update date in copyright header.
        * gdb/gnulib/configure.ac: Ditto.
        * gdb/gnulib/update-gnulib.sh: Ditto.