package/mtr: uses fork(), needs mmu

Fixes
http://autobuild.buildroot.net/results/dcb/dcbd39878b1a80d6a16032e967bd8e2b928f7a78/
http://autobuild.buildroot.net/results/e05/e05632e0b160ee995ce46aaf3ceddf46d3658adf/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mtr/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_MTR
 	bool "mtr"
+	depends on BR2_USE_MMU # fork()
 	help
 	  mtr combines the functionality of the 'traceroute' and 'ping'
 	  programs in a single network diagnostic tool.