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/twrp-functions.cpp b/twrp-functions.cpp
index bc17c88..945f61d 100755
--- a/twrp-functions.cpp
+++ b/twrp-functions.cpp
@@ -1396,6 +1396,7 @@
 #endif
 }
 
+#ifdef USE_EXT4
 bool TWFunc::Get_Encryption_Policy(ext4_encryption_policy &policy, std::string path) {
 #ifdef TW_INCLUDE_FBE
 	if (!TWFunc::Path_Exists(path)) {
@@ -1426,6 +1427,7 @@
 #endif
 	return true;
 }
+#endif
 
 string TWFunc::Check_For_TwrpFolder(){
 	string oldFolder = "";