apf9328: Fix build error

Commit a0b6faaa ("gdb: convert to the package infrastructure") changed
the name of the host gdb configuration option, and added it to the
legacy option set, leading to a build breakage for the
armadeus_apf9328_defconfig.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>