exec: Restrict memory-internal.h to system/

exec: Restrict memory-internal.h to system/

Only file units within the system/ directory need access to
"memory-internal.h". Restrict its scope by moving it there.

The comment from commit 9d70618c684 ("memory-internal.h:
Remove obsolete claim that header is obsolete") is now obsolete,
remove it.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250317161329.40300-3-philmd@linaro.org>