vsftpd: needs mmu

Fixes
http://autobuild.buildroot.net/results/067cfbab9fa26e9e5f321f661e8f4ac0e19249e1

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

package/vsftpd/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_VSFTPD
 	bool "vsftpd"
+	depends on BR2_TOOLCHAIN_HAS_THREADS # fork()
 	help
 	  vsftpd is an ftp daemon written with security in mind.
 	  http://vsftpd.beasts.org/