tmux: needs mmu

Fixes:
http://autobuild.buildroot.net/results/1b2/1b2a8c75e73ec136e639896f23544bf85906e97b/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/tmux/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_TMUX
 	bool "tmux"
+	depends on BR2_USE_MMU # fork()
 	select BR2_PACKAGE_LIBEVENT
 	select BR2_PACKAGE_NCURSES
 	help