FIX build for Windows executable

[lpg2709]

When I build (for Windows) the version from commit a04d6d9b3995eabe26a41c9d5abc56678dd8b035 the final executable show the follow error if you try to execute:

Traceback (most recent call last):
  File "nanovna-saver.py", line 32, in <module>
  File "PyInstaller\loader\pyimod02_importers.py", line 419, in exec_module
  File "NanoVNASaver\__main__.py", line 31, in <module>
ModuleNotFoundError: No module named 'PyQt6.sip'

With this changes on build, the pyinstaller find correctly the Qt6 dependency and link on the final executable.

Pull Request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

pyinstaller build the executable, but can't correct link the PyQt6.sip dependency.

Issue Number: 1

What is the new behavior?

pyinstaller find PyQt6.sip and Qt6 dependency, correctly.

Does this introduce a breaking change?

• Yes
• No

Other information

This change only affects the Windows compilation, may be useful for Linux and MacOS, when it was in interest to FIX their build.