.gitignore: don;t accidentally ignore some files

.gitignore: don;t accidentally ignore some files

The previous first line of .gitignore ("bin/") correctly ignored the
bin/ directory, but it also ignored the git controlled content in
source4/scripting/bin.  The correct line is "/bin/", as explained in
these snippets from the gitignore documentation:

    If the pattern ends with a slash, it is removed for the purpose
    of the following description, but it would only find a match with
    a directory. In other words, foo/ will match a directory foo and
    paths underneath it, but will not match a regular file or a
    symbolic link foo (this is consistent with the way how pathspec
    works in general in Git).

    A leading slash matches the beginning of the pathname. For example,
    "/*.c" matches "cat-file.c" but not "mozilla-sha1/sha1.c".

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>