ext4crypt: only include if TARGET_USERIMAGES_USE_EXT4 is set

Fixes build error on legacy devices that don't have this defined

Change-Id: Iab74c5ab952bf1d142d981823ab67867a6f7d185
diff --git a/partitions.hpp b/partitions.hpp
index 637471b..58fcd81 100755
--- a/partitions.hpp
+++ b/partitions.hpp
@@ -26,7 +26,10 @@
 #include "exclude.hpp"
 #include "tw_atomic.hpp"
 #include "progresstracking.hpp"
+
+#ifdef USE_EXT4
 #include "ext4crypt_tar.h"
+#endif
 
 #define MAX_FSTAB_LINE_LENGTH 2048