bash: add depends on MMU

Fixes the build failure at

  http://autobuild.buildroot.org/results/0434d99b51f535ac73bfcf1968b1c2b81b44ba61/build-end.log.

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

package/bash/Config.in

@@ -1,6 +1,8 @@
 config BR2_PACKAGE_BASH
 	bool "bash"
 	select BR2_PACKAGE_NCURSES
+	# uses fork()
+	depends on BR2_USE_MMU
 	help
 	  The standard GNU Bourne again shell.