Convert fscrypt_set_directory_policy to C++

Bug: 140027478
Test: compiles, boots
Change-Id: Ic0c8b163fe37b11787cab49cc2eea38a1de377e9
diff --git a/init/builtins.cpp b/init/builtins.cpp
index c3bdc53..fb10a87 100644
--- a/init/builtins.cpp
+++ b/init/builtins.cpp
@@ -399,7 +399,7 @@
     }
 
     if (fscrypt_is_native()) {
-        if (fscrypt_set_directory_policy(args[1].c_str())) {
+        if (fscrypt_set_directory_policy(args[1])) {
             return reboot_into_recovery(
                 {"--prompt_and_wipe_data", "--reason=set_policy_failed:"s + args[1]});
         }