nconfig: mark local functions as such

scripts/kconfig/nconf.gui.c:23: warning: no previous prototype for 'set_normal_colors'
scripts/kconfig/nconf.gui.c:68: warning: no previous prototype for 'normal_color_theme'
scripts/kconfig/nconf.gui.c:100: warning: no previous prototype for 'no_colors_theme'
scripts/kconfig/nconf.c:455: warning: no previous prototype for 'process_special_keys'
scripts/kconfig/nconf.c:487: warning: no previous prototype for 'get_next_hot'
scripts/kconfig/nconf.c:506: warning: no previous prototype for 'canbhot'
scripts/kconfig/nconf.c:514: warning: no previous prototype for 'is_hot'
scripts/kconfig/nconf.c:522: warning: no previous prototype for 'make_hot'
scripts/kconfig/nconf.c:582: warning: no previous prototype for 'item_make'
scripts/kconfig/nconf.c:626: warning: no previous prototype for 'item_add_str'
scripts/kconfig/nconf.c:656: warning: no previous prototype for 'item_tag'
scripts/kconfig/nconf.c:668: warning: no previous prototype for 'curses_item_index'
scripts/kconfig/nconf.c:673: warning: no previous prototype for 'item_data'
scripts/kconfig/nconf.c:684: warning: no previous prototype for 'item_is_tag'
scripts/kconfig/nconf.c:691: warning: no previous prototype for 'set_config_filename'

Signed-off-by: Michal Marek <mmarek@suse.cz>