Add a missing #include <version> to Core.

Reference issue

#2986 (closed)

What does this implement/fix?

This MR adds a missing #include <version> to the Core header. At present, #include <bit> is gated behind feature test against the macro __cpp_lib_bit_cast however per the standards this macro is only defined in the version header. See https://en.cppreference.com/w/cpp/feature_test.html.

Note that while NumTraits.h also makes its call to std::bit_cast conditional on __cpp_lib_bit_cast , downstream source files may have an #include <version> after it's already included Eigen, triggering a call to std::bit_cast without bit being included and thus a compile error.