index.c: Fix a sparse "symbol not declared" warning
In particular, sparse issues a warning about the declaration
of git_index__parse_tree(). However, I suspect that, at some
point, the git_index__parse_treecache() function (which does
not have a definition) was renamed and the declaration was
missed. In order to suppress this warning, we simply change
the name of the function in the declaration from git_index\
__parse_treecache() to git_index__parse_tree().
Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>