[gdb] Fix leftover conflict marker in gdb/config/djgpp/README

For gdb/config/djgpp/README, we have:
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gdb/config/djgpp/README:6: leftover conflict marker
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This is a false positive, this is just some text formatting using '=':
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General
=======
...
whose length happens to match the default value of git attribute
conflict-marker-size: 7.

Fix this by setting the conflict-marker-size to 80.

Approved-By: Tom Tromey <tom@tromey.com>