kconfig/conf: mark xfgets() private

This function has not much reason to be public. In the mean time, convert
declaration from K&R C to ISO C.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>

scripts/kconfig/lkc.h

@@ -76,9 +76,6 @@ void zconf_nextfile(const char *name);
 int zconf_lineno(void);
 const char *zconf_curname(void);
 
-/* conf.c */
-void xfgets(char *str, int size, FILE *in);
-
 /* confdata.c */
 const char *conf_get_configname(void);
 const char *conf_get_autoconfig_name(void);