cups: uses fork(), only available on MMU platforms

Fixes
http://autobuild.buildroot.net/results/6db1eb27387696bfa6b4d8ffb30eff00a2f842d0/build-end.log.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/cups/Config.in

@@ -1,5 +1,7 @@
 config BR2_PACKAGE_CUPS
 	bool "cups"
+	# needs fork()
+	depends on BR2_USE_MMU
 	help
 	  The Common Unix Printing System