command.h: Remove extern from the header

Remove the extern of the header because they are useless.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[bmeng: minor edit on the commit message]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>