nuttcp: needs mmu

Fixes:
http://autobuild.buildroot.net/results/b8c019a5701377a1e134d01ac0bae5fe34dc975f/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/nuttcp/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_NUTTCP
 	bool "nuttcp"
+	depends on BR2_USE_MMU # fork()
 	help
 	  nuttcp is a TCP/UDP network testing tool, much like iperf