gdb/copyright.py: Do not forget to remind about MULTIPLE_COPYRIGHT_HEADERS

gdb/copyright.py: Do not forget to remind about MULTIPLE_COPYRIGHT_HEADERS

There is a small logical error in the part of the script that
dumps the list of files in BY_HAND + MULTIPLE_COPYRIGHT_HEADERS
but only checkis the contents of BY_HAND. The issue becomes
apparent as soon as BY_HAND is empty. Prevent this from happening
by treating the two lists separately, as this allows us to provide
a more informative message in the case of MULTIPLE_COPYRIGHT_HEADERS.

gdb/ChangeLog:

        * copyright.py (main): Dump the contents of
        MULTIPLE_COPYRIGHT_HEADERS (separately) from BY_HAND,
        even if BY_HAND is empty.