debug/gdb: fix $need_ncurses_src usage

It should be used only to decide whether we need to download/extract
ncurses, not wheter we should build it or not.

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/"

 reporter_comment="Toolchain for the Raspberry Pi, with hard-float."