CONFIG_NEEDS_MANUAL_RELOC: Fix warnings when not set

Now that we may compile (but not link) code calling fixup_cmdtable when
this is not set, we need to always have the declaration available.  We
should also make sure that anyone calling the function includes
<command.h> as that's where the function declaration is.

Signed-off-by: Tom Rini <trini@konsulko.com>