dialog: add missing comment when thread support is missing

This was noticed when reviewing the packaging for the python-dialog
package.

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

package/dialog/Config.in

@@ -10,3 +10,7 @@ config BR2_PACKAGE_DIALOG
 	  This application provides a method of displaying several different
 	  types of dialog boxes from shell scripts.  This allows a developer
 	  of a script to interact with the user in a much friendlier manner.
+
+comment "dialog needs a toolchain w/ threads"
+	depends on !BR2_TOOLCHAIN_HAS_THREADS
+	depends on BR2_USE_MMU