wafsamba/samba_utils.py: override symlink to allow force link

wafsamba/samba_utils.py: override symlink to allow force link

if bin is not empty and I have been sharing the samba tree into
a Vagrant environment and we run make, we get annoying linking error like this:

     File "~/samba/lib/tevent/wscript", line 130, in build
        installdir='python')
      File "./buildtools/wafsamba/wafsamba.py", line 745, in SAMBA_SCRIPT
        os.symlink(link_src, link_dst)
    FileExistsError: [Errno 17] File exists: '~/samba/lib/tevent/tevent.py' -> '~/samba/bin/default/../python/tevent.py'
    Makefile:7: recipe for target 'all' failed

Override the symlink method to allow force linking.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>