xapp_xsm: depends on MMU

Fixes: http://sysmic.org/~jezz/results/455f2788:

remote.o: In function `_remote_start':
remote.c:(.text+0x74): undefined reference to `_fork'
restart.o: In function `_Clone':
restart.c:(.text+0x5a8): undefined reference to `_fork'
restart.o: In function `_Restart':
restart.c:(.text+0xa44): 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_xsm/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_XAPP_XSM
 	bool "xsm"
+	depends on BR2_USE_MMU # fork()
 	select BR2_PACKAGE_XLIB_LIBXAW
 	help
 	  X Session Manager