package/ncdu: needs MMU

It uses the system() function, which calls fork().

Fixes:
    http://autobuild.buildroot.org/results/642/642f408d384a32033a677d56e6e3cdf572bd798b/
    http://autobuild.buildroot.org/results/026/026657e624928ed6bede5719f96c2eee797076cf/
    http://autobuild.buildroot.org/results/86b/86b66ed92fb714795ba70c228d4d448a1b61cee0/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/ncdu/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_NCDU
 	bool "ncdu"
+	depends on BR2_USE_MMU # system()
 	select BR2_PACKAGE_NCURSES
 	help
 	  ncdu is a disk usage analyzer with an ncurses interface