Add missing <optional> include

[chenrui333]

Adds the missing <optional> include to peglib.h.

Ref: Homebrew/homebrew-core#271855

[yhirose]

@chenrui333 thanks for the report! I am a bit curious because GitHub Actions workflow shows no problem to build.

[chenrui333]

I force-pushed this branch to a single repro-only commit that adds a standalone #include <peglib.h> compile target.

This reproduces the Homebrew GCC failure on my side with g++-15: the new target fails because peglib.h uses std::optional without including <optional>.

The reason the current GitHub Actions workflow stayed green is that the test TUs include gtest/gtest.h before peglib.h, and GoogleTest itself pulls in <optional>, which masks the missing direct include.

Once this repro is clear, I can stack the one-line <optional> fix back on top.

[yhirose]

@chenrui333, thank you for the detailed information. I'm fine with the original change containing just the <optional> include. Could you revert your pull request to its original state?

[chenrui333]

yeah, totally.

[chenrui333]

updated.

[yhirose]

@chenrui333 thanks for your contribution!