bootcount_ext: Add Ext4 build dependency

This config option depends on EXT4 support.
If Ext4 write capability is not selected, it generate write error messages
and is unable to maintain boot counter.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
diff --git a/drivers/bootcount/Kconfig b/drivers/bootcount/Kconfig
index 0356f8b..c8e6fa7 100644
--- a/drivers/bootcount/Kconfig
+++ b/drivers/bootcount/Kconfig
@@ -27,6 +27,8 @@
 
 config BOOTCOUNT_EXT
 	bool "Boot counter on EXT filesystem"
+	depends on FS_EXT4
+	select EXT4_WRITE
 	help
 	  Add support for maintaining boot count in a file on an EXT
 	  filesystem.