Fix clang -Wdeprecated-copy.

The copy constructor must be explicitly defined (here, defaulted) to be
available when the assignment operator is deleted.  While there is a
templated constructor from VmaStlAlloc<U>, the compiler doesn't consider
this to be a "copy" constructor for purposes of the warning.

include/vk_mem_alloc.h

@@ -4963,6 +4963,7 @@ public:
     }
 
     VmaStlAllocator& operator=(const VmaStlAllocator& x) = delete;
+    VmaStlAllocator(const VmaStlAllocator&) = default;
 };
 
 #if VMA_USE_STL_VECTOR