heirloom-mailx: needs fork()

heirloom-mailx uses fork(), so we can't use it as is on !mmu
platforms.

Fixes:

http://autobuild.buildroot.org/results/c5a8e3a4cbc1e1de985fe3c724b711027865abe7/build-end.log

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/heirloom-mailx/Config.in

@@ -1,5 +1,7 @@
 config BR2_PACKAGE_HEIRLOOM_MAILX
 	bool "heirloom-mailx"
+	# uses fork()
+	depends on BR2_USE_MMU
 	help
 	  Heirloom mailx (previously known as nail) is a mail user
 	  agent for Unix systems.