powerpc/windfarm: Fix XServe G5 fan control Makefile issue

We are missing building windfarm_max6690_sensor.o when building
CONFIG_WINDFARM_RM31. Usually all the windfarm drivers are built
and thus this isn't a problem but some more "tailored" setups
(Gentoo ?) building only that driver are not working because
the require sensor module is missing.

Reported-by: Stanislav Ponomarev <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

drivers/macintosh/Makefile

···
 				   windfarm_ad7417_sensor.o \
 				   windfarm_lm75_sensor.o \
 				   windfarm_lm87_sensor.o \
+				   windfarm_max6690_sensor.o \
 				   windfarm_pid.o \
 				   windfarm_cpufreq_clamp.o \
 				   windfarm_rm31.o