kconfig: fix sparse warnings in nconfig

Fix sparse warnings in scripts/kconfig/nconf* ('make nconfig'):

../scripts/kconfig/nconf.c:1071:32: warning: Using plain integer as NULL pointer
../scripts/kconfig/nconf.c:1238:30: warning: Using plain integer as NULL pointer
../scripts/kconfig/nconf.c:511:51: warning: Using plain integer as NULL pointer
../scripts/kconfig/nconf.c:1460:6: warning: symbol 'setup_windows' was not declared. Should it be static?
../scripts/kconfig/nconf.c:274:12: warning: symbol 'current_instructions' was not declared. Should it be static?
../scripts/kconfig/nconf.c:308:22: warning: symbol 'function_keys' was not declared. Should it be static?
../scripts/kconfig/nconf.gui.c:132:17: warning: non-ANSI function declaration of function 'set_colors'
../scripts/kconfig/nconf.gui.c:195:24: warning: Using plain integer as NULL pointer

nconf.gui.o before/after files are the same.
nconf.o before/after files are the same until the 'static' function
declarations are added.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>