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: NStanislav Ponomarev <devhexorg@gmail.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

drivers/macintosh/Makefile

@@ -40,6 +40,7 @@ obj-$(CONFIG_WINDFARM_RM31)     += windfarm_fcu_controls.o \
 				   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