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

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".