empty: needs MMU

The empty package uses the fork() system call, and so requires MMU.

Fixed autobuilder failures such as the following.

  http://autobuild.buildroot.net/results/4ad23e07b1bfe7bfd622aed8d54fa243b72eed59/build-end.log

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/empty/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_EMPTY
 	bool "empty"
+	depends on BR2_USE_MMU # uses fork()
 	help
 	  Run processes and applications under pseudo-terminal (PTY) sessions.