paged-kv-manager.h: add missing <cstddef> for size_t

Fixes cuda-13 amd64 / non-arm64 build where size_t was used without the
header (arm64 cuda-13 pulled it in transitively; amd64/cuda-12 toolchains
do not). Compile-only change, bit-exactness unaffected.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:opus-4.8 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

backend/cpp/llama-cpp-localai-paged/patches/paged/0001-vendor-paged-kv-manager.patch

@@ -333,7 +333,7 @@ new file mode 100644
 index 000000000..740280a7f
 --- /dev/null
 +++ b/src/paged-kv-manager.h
-@@ -0,0 +1,108 @@
+@@ -0,0 +1,109 @@
 +#pragma once
 +// Paged KV cache block manager for llama.cpp (CPU-first prototype).
 +//
@@ -346,6 +346,7 @@ index 000000000..740280a7f
 +// LRU eviction order), not on exact hash bytes. This unit has zero ggml/llama.cpp
 +// dependency so it can be unit-tested in isolation.
 +
++#include <cstddef>
 +#include <cstdint>
 +#include <vector>
 +#include <unordered_map>