gdb: needs host-ncurses on the host

Commit 2474fb0b ("host-gdb: enable
terminal user interface support") has added TUI support to host gdb,
and therefore added a dependency on host-ncurses when TUI support is
enabled.

However, host-ncurses is not only needed for TUI support, it is needed
for gdb in all cases as well, so this commit adds a dependency of
host-gdb to host-ncurses.

Fixes:

  http://autobuild.buildroot.org/results/153/153dbdc42103074f7a0895e8871e2eee4eae3325/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>