proftpd: needs mmu

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/proftpd/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_PROFTPD
 	bool "proftpd"
+	depends on BR2_USE_MMU # fork()
 	help
 	  ProFTPD, a highly configurable FTP server.