squashfs: add comment mmu depends

Otherwise it shows up indirectly when toolchain options aren't enough
and then vanishes when they are fulfilled.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/squashfs/Config.in

@@ -46,3 +46,4 @@ endif
 
 comment "squashfs needs a toolchain w/ largefile, threads"
 	depends on !BR2_LARGEFILE || !BR2_TOOLCHAIN_HAS_THREADS
+	depends on BR2_USE_MMU