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="Thomas Petazzoni"

 reporter_url="http://www.free-electrons.com/"

 reporter_comment="Very basic testing only"