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="Kevin Cernekee"

reporter_url="http://sourceware.org/ml/crossgcc/2011-09/msg00037.html"

reporter_comment="MIPS SDE-compatible toolchain"