Fix cross-compile ntpd 4.2.6: keyword-gen: cannot execute binary file

To fix the error:

./keyword-gen ./ntp_parser.h > k-g.out
/bin/bash: ./keyword-gen: cannot execute binary file

Change the ntpd/Makefile.am to:

k-g-u-submake: keyword-gen
if NTP_CROSSCOMPILE_FALSE
./keyword-gen $(srcdir)/ntp_parser.h > k-g.out
@grep -v diff_ignore_line < k-g.out > cmp1
@grep -v diff_ignore_line < $(srcdir)/ntp_keyword.h > cmp2
@cmp cmp1 cmp2 > /dev/null || \
{ mv -f k-g.out $(srcdir)/ntp_keyword.h && \
echo ‘Generated changed ntp_keyword.h.’ ;}
@[ ! -f k-g.out ] || \
{ rm k-g.out && echo ‘ntp_keyword.h is up to date.’ ;}
@rm cmp1 cmp2
@echo ‘keyword-gen and ntp_keyword.h are up to date.’ > $@
endif

keyword-gen: cannot execute binary file