fan-ctrl: needs MMU support

Fixes http://autobuild.buildroot.org/results/333/333b6caa6b8a9a04495d82d02b2c0fb7755667ed/

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/fan-ctrl/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_FAN_CTRL
 	bool "fan-ctrl"
+	depends on BR2_USE_MMU # fork()
 	help
 	  Fan-ctrl is a little daemon that controls the speed of one
 	  or more system or CPU fans. It does so by monitoring