windfarm: make wf_critical_overtemp() static

wf_critical_overtemp() is exported. But nothing uses that export.
That's unsurprising because there's no header that defines it. Stop
exporting that function and make it static.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>