allow new files in include/LightGBM

[jameslamb]

This fixes an unintended behavior in this project's git configuration.

The .gitignore rule intended to block inclusion of the LightGBM CLI (a binary file named lightgbm) also prevents including of any new files in include/LightGBM:

touch include/LightGBM/something.h
git add include/LightGBM/something.h

The following paths are ignored by one of your .gitignore files:
include/LightGBM
hint: Use -f if you really want to add them.
hint: Turn this message off by running
hint: "git config advice.addIgnoredFile false"

@borchero ran into this here: #6034 (comment)

This PR fixes that by adding a leading / to that rule. References explaining why that works:

• https://stackoverflow.com/a/6168768/3986677
• https://git-scm.com/docs/gitignore#_pattern_format

[github-actions]

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