Both os-win32.h and os-posix.h include system header files. Instead
of having osdep.h include them inside its 'extern "C"' block, make
these headers handle that themselves, so that we don't include the
system headers inside 'extern "C"'.

This doesn't fix any current problems, but it's conceptually the
right way to handle system headers.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>