common: fix missing function pointer relocation in fixup_cmdtable()

The command auto-completion does not work on architectures relying
on CONFIG_NEEDS_MANUAL_RELOC like MIPS. Cause is the missing function
pointer fixup for cmd_tbl_t::complete function in fixup_cmdtable().

This patch adds the missing pointer fixup in case of CONFIG_AUTO_COMPLETE
is defined.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>