buildroot

package/gdb: switch to 7.11 as the default version

7.12 is around, 7.11 has already seen a point release, so it's time to
make 7.11 the default version for gdb.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>