kconfig: use xfwrite wrapper function to silence warnings

Use the xfwrite wrapper function defined in lkc.h to check the return value of
fwrite and silence these warnings.

  HOSTCC  scripts/kconfig/zconf.tab.o
config/zconf.tab.c: In function 'header_print_comment':
config/confdata.c:551:10: warning: ignoring return value of 'fwrite', declared with attribute warn_unused_result
config/zconf.tab.c: In function 'kconfig_print_comment':
config/confdata.c:467:10: warning: ignoring return value of 'fwrite', declared with attribute warn_unused_result

Signed-off-by: Peter Foley <pefoley2@verizon.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>