Don't use std::allocator::pointer

It's removed in C++20

Bug: 175635923
Test: m MODULES-IN-system-vold
Change-Id: Ief2875bfd3e2d2e5023ad4c0bb754a616fd42419
diff --git a/KeyBuffer.h b/KeyBuffer.h
index 3275255..619cb27 100644
--- a/KeyBuffer.h
+++ b/KeyBuffer.h
@@ -28,7 +28,7 @@
 // Allocator that delegates useful work to standard one but zeroes data before deallocating.
 class ZeroingAllocator : public std::allocator<char> {
   public:
-    void deallocate(pointer p, size_type n) {
+    void deallocate(value_type* p, size_type n) {
         memset_explicit(p, 0, n);
         std::allocator<char>::deallocate(p, n);
     }