devel/shiboken2: Bump supported Python version

Shiboken2 5.15.11 has initial support for Python 3.11 and this will
help the python@ team transition to Python 3.11 as the default.

While here, add the noexport flag to USES=llvm to allow building with
ccache.

Reported by: wen (via the kde-freebsd mailing list)