[pyinstaller] Add missing compat.is_netbsd - #16217

Fixes the stubtest failure reported in #16213.

PyInstaller.compat defines is_netbsd next to the other platform flags, but the stub does not have it, so the daily run fails with:

error: PyInstaller.compat.is_netbsd is not present in stub
Stub: in file stubs/pyinstaller/PyInstaller/compat.pyi
MISSING
Runtime:
False

Upstream defines it between is_openbsd and is_hpux, and also folds it into is_unix (compat.py#L90-L97):

is_freebsd = sys.platform.startswith('freebsd')
is_openbsd = sys.platform.startswith('openbsd')
is_netbsd = sys.platform.startswith('netbsd')
is_hpux = sys.platform.startswith('hp-ux')
...
is_unix = is_linux or is_solar or is_aix or is_freebsd or is_hpux or is_openbsd or is_netbsd

I put the new line in the same position so the stub keeps matching that order.

Verified locally against pyinstaller==6.22.1 with the pinned mypy==2.3.0. Before the change stubtest reports the error above; after it:

Success: no issues found in 165 modules

One note in case it comes up in CI: ruff format --diff wants to remove two blank lines further down the same file, around the getenv overloads. That is already the case on main without this change, so I left it alone rather than mixing an unrelated reformat into this fix.