.gitignore: ignore any __BASEDIR__ symlinks

.gitignore: ignore any __BASEDIR__ symlinks

Later perls do not contain the current directory (".") by default
in their include path.

In order to successfully run `perl -c` on the various perl scripts,
they must be able to find their include files.

Prior to install they contain a `use lib "__BASEDIR__"` line that
gets modified at install time to reflect the actual location of the
include base directory.

In order to use `perl -c` on the uninstalled versions, an explicit
`-I.` argument must be provided.

This can get especially tedious after a while.

An alternative is to create a symlink from __BASEDIR__ in the current
directory to `.` (or whereever the root of the include tree is
actually located).

While quite convenient, it's something to be added by individual
developers according to their preference rather than just blindly
adding such a symlink to the repository thereby affecting everyone.

However, to facilitate such use, it doesn't hurt to add an ignore
pattern for `__BASEDIR__` to the top-level .gitignore to help prevent
any possible accidents where such a link might get added and committed
unintentionally.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>