libvncserver: needs mmu

Fixes http://autobuild.buildroot.net/results/30e/30e6a879ab6d84df4688c3d29761522f68dad9fc/

fork() is only used in VNCommand.c, so it seems doable to patch the sources
(and send upstream) to make it work on !mmu if people are interested.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libvncserver/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_LIBVNCSERVER
 	bool "libvncserver"
+	depends on BR2_USE_MMU # VNCommand.c uses fork()
 	help
 	  libvncserver is a VNC server/client library.