package/wiringpi: uses fork(), needs mmu

Fixes
http://autobuild.buildroot.net/results/c60/c6013c214674f9802309d8cd770d3da2f751674d/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/wiringpi/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_WIRINGPI
 	bool "wiringpi"
+	depends on BR2_USE_MMU # fork()
 	help
 	  wiringPi libraries (and gpio command)