thttpd: needs mmu

Fixes
http://autobuild.buildroot.net/results/0b73d2b45a5bac94b65893725de46d87a9626b37/

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

package/thttpd/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_THTTPD
 	bool "thttpd"
+	depends on BR2_USE_MMU # fork()
 	help
 	  thttpd is a simple, small, portable, fast, and secure HTTP server