package/rtty: needs MMU

rtty uses fork(), so add a depends on BR2_USE_MMU

Fixes:

  http://autobuild.buildroot.net/results/ebd483ee87046396db39f966e34f93c3d1123202/

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

package/rtty/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_RTTY
 	bool "rtty"
+	depends on BR2_USE_MMU # fork()
 	select BR2_PACKAGE_LIBEV
 	help
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