xapp_xdm: depends on MMU

Fixes http://sysmic.org/~jezz/results/e891eb7e:

dm.o: In function `_StartDisplay':
dm.c:(.text+0xd6e): undefined reference to `_fork'
server.o: In function `_StartServerOnce':
server.c:(.text+0x266): undefined reference to `_fork'
session.o: In function `_runAndWait':
session.c:(.text+0x67a): undefined reference to `_fork'
session.o: In function `_ManageSession':
session.c:(.text+0xa2a): undefined reference to `_fork'
collect2: ld returned 1 exit status

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/x11r7/xapp_xdm/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_XAPP_XDM
 	bool "xdm"
+	depends on BR2_USE_MMU # fork()
 	# Runtime dependency: can't start an X server unless there is one
 	depends on BR2_PACKAGE_XSERVER_XORG_SERVER
 	select BR2_PACKAGE_XAPP_SESSREG