platform/x86: DELL_WMI use depends on instead of select for DELL_SMBIOS

If DELL_WMI "select"s DELL_SMBIOS, the DELL_SMBIOS dependencies are
ignored and it is still possible to end up with unmet direct
dependencies.

Change the select to a depends on.

Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>