um: enable VMAP_STACK

This works just fine, so select HAVE_ARCH_VMAP_STACK to
let users enable VMAP_STACK if desired.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/Kconfig

@@ -21,6 +21,7 @@ config UML
 	select HAVE_GCC_PLUGINS
 	select SET_FS
 	select TTY # Needed for line.c
+	select HAVE_ARCH_VMAP_STACK
 
 config MMU
 	bool