ncftp: specify ln -sf to remove existing destination files

This fixes below build error when executing "make ncftp-dirclean; make".

ln -s /usr/bin/ncftpbatch /opt/test/br-x86/buildroot/output/target/usr/bin/ncftpspooler
ln: failed to create symbolic link '/opt/test/br-x86/buildroot/output/target/usr/bin/ncftpspooler': File exists
make: *** [/opt/test/br-x86/buildroot/output/build/ncftp-3.2.5/.stamp_target_installed] Error 1

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>