[branch+head] Mention GDB 7.3.1 release in ChangeLog

[Joel Brobecker]

Hello,

Now that GDB 7.3.1 is out, I have added the following GDB ChangeLog
entry, to document the time of creation of this release.

2011-09-04  Joel Brobecker  <brobecker@adacore.com>

        GDB 7.3.1 released.

Checked in (branch+head).

Thanks,
-- 
Joel