gatekeeper: fix new[]/delete mismatch

UniquePtr<T[]> uses delete[], whereas UniquePtr<T> uses delete. Since
we're using new[], we should be using delete[].

Caught by clang's static analyzer.

Bug: 27101951
Test: mma. Static analyzer warning is gone.
Change-Id: I30a72b8c111764f602aeee7c09fc2ce781ad58cb
diff --git a/gatekeeper.cpp b/gatekeeper.cpp
index cfd878f..3b5868e 100644
--- a/gatekeeper.cpp
+++ b/gatekeeper.cpp
@@ -182,7 +182,7 @@
 
     uint32_t metadata_length = sizeof(user_id) + sizeof(flags) + sizeof(HANDLE_VERSION);
     const size_t to_sign_size = password_length + metadata_length;
-    UniquePtr<uint8_t> to_sign(new uint8_t[to_sign_size]);
+    UniquePtr<uint8_t[]> to_sign(new uint8_t[to_sign_size]);
 
     if (to_sign.get() == nullptr) {
         return false;