gdb: fix target gdb building from git

gdb: fix target gdb building from git

Currently we only add "host-texinfo" dependency for host GDB built
from git sources. But in case of target gdb texinfo won't be built
and so gdb building will fail on attempt to build documentation.

Fix is trivial - add "host-texinfo" as a dependency for target gdb
in the sme way as we disable documentation building via both
GDB_CONF_ENV (for target) and HOST_GDB_CONF_ENV (host) GDB flavors.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Anton Kolesov <akolesov@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>