fix gdb version parsing in src-release.sh

Small change required after we switched the gdb version scheme to
using a -git suffix rather than a -cvs one.

ChangeLog:

        * src-release.sh: Compute the gdb tarball name by stripping
        '-git' rather than '-cvs'.

Tested by running "src-release.sh gdb" and verifying the tarball
name as well as its contents.