nconfig: Silence unused return values from wattrset

Ignore the return value from wattrset since we ignore the return
value in nconf.gui.c as well.

scripts/kconfig/nconf.c: In function 'print_function_line':
scripts/kconfig/nconf.c:376: warning: value computed is not used
scripts/kconfig/nconf.c:380: warning: value computed is not used
scripts/kconfig/nconf.c:387: warning: value computed is not used
scripts/kconfig/nconf.c: In function 'show_menu':
scripts/kconfig/nconf.c:956: warning: value computed is not used
scripts/kconfig/nconf.c:961: warning: value computed is not used
scripts/kconfig/nconf.c:963: warning: value computed is not used
scripts/kconfig/nconf.c:965: warning: value computed is not used

Cc: Nir Tzachar <nir.tzachar@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>