86Box nightly – autobuild-70c70c0

Fix Linux build failure from cs/gs macro collision with csignal

Move #include <csignal> before the extern "C" block containing cpu.h
to prevent the cs/gs macros (defined in cpu.h) from expanding inside
struct sigcontext in Linux signal headers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>