package/gdb: improve gdb version choice comment

package/gdb: improve gdb version choice comment

The comment has been introduced by commit [1] where the latest
gdb version has been used when cross-gdb is not enabled.

But since then the gdb package doesn't use the latest gdb version when
cross-gdb is not enabled. It's the "stable" version.

[1] https://git.buildroot.net/buildroot/commit/?id=fda818390b5e6a585608f4523356eafa0c587f53

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>