babeld: needs MMU as it uses fork()

Fixes:

  http://autobuild.buildroot.net/results/986953a9bb49cd68338dc15e1a2f96045e44bf02/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/babeld/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_BABELD
 	bool "babeld"
+	depends on BR2_USE_MMU # fork()
 	help
 	  Babel is a loop-avoiding distance-vector routing protocol for IPv6
 	  and IPv4 with fast convergence properties.