package/sexpect: requires MMU

Disable when MMU is not available for target.

Fixes:
http://autobuild.buildroot.net/results/a2b69fdf67d520aa4d2718bd688d88d102842cdf

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/sexpect/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_SEXPECT
 	bool "sexpect"
+	depends on BR2_USE_MMU # fork()
 	help
 	  sexpect is another implementation of Expect which is
 	  specifically designed for Shell scripts