Add C++ ifdefs for cgreen_value_internal.h
Add C++ ifdefs for cgreen_value_internal.h

We do compile our own tests with C++ to ensure that it works, so even
internal header files need the C++ "namespace" prelude

thoni56/cgreen,cgreen-devs/cgreen,thoni56/cgreen,thoni56/cgreen,thoni56/cgreen,thoni56/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen