fix: make gil.h self-contained for PyPy/GraalPy and silence -Wredundant-decls

gil.h evaluated PYBIND11_SIMPLE_GIL_MANAGEMENT before including common.h,
which defines it on PyPy/GraalPy. Every existing TU included common.h
first through pybind11.h, so this only surfaced when src/type_caster_base.cpp
reached gil.h directly. Also suppress GCC -Wredundant-decls for the
isinstance_generic declaration duplicated in pytypes.h.

Assisted-by: ClaudeCode:claude-fable-5