gdb/copyright.py: Add generated files in gnulib/ to exclude list

This will prevent this script from updating the copyright year range
for those files.

Note that aclocal.m4 and configure are already in the EXCLUDE_ALL_LIST,
so they don't need to be added to the EXCLUDE_LIST.

gdb/ChangeLog:

        * copypright.py (EXCLUDE_LIST): Add 'gnulib/config.in' and
        'gnulib/Makefile.in' to the list.