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>

 CT_EXPERIMENTAL=y

 CT_LOCAL_TARBALLS_DIR="${HOME}/src"

 CT_SAVE_TARBALLS=y

 CT_CONFIG_SHELL_SH=y

 CT_LOG_EXTRA=y

 CT_ARCH_avr32=y

 CT_BINUTILS_V_2_18a=y

 CT_CC_V_4_2_2=y

 CT_LIBC_NEWLIB_V_1_17_0=y