sysinfo: Move #ifdef so that operations are always defined

At present the struct is not available unless SYSINFO is enabled. This is
annoying since code it is not possible to use compile-time checks like
CONFIG_IS_ENABLED(SYSINFO) with this header.

Fix it by moving the #ifdef.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>