llama-batch: fix build fails with -Werror=missing-braces (#16614)

## Why it failed

When compiling with strict compiler flags (-Wmissing-braces -Werror=missing-braces),
the build fails with the following error:

```
cmake \
  -S . \
  -B ../llama.cpp.build \
  --preset=x64-linux-gcc-debug \
  -DCMAKE_INSTALL_PREFIX=/tmp/local \
  -DCMAKE_CXX_FLAGS="-Wmissing-braces -Werror=missing-braces" && \
cmake --build ../llama.cpp.build/
...
In file included from /home/otegami/work/cpp/llama.cpp/src/llama-graph.h:4,
                 from /home/otegami/work/cpp/llama.cpp/src/llama-model.h:5,
                 from /home/otegami/work/cpp/llama.cpp/src/llama.cpp:8:
/home/otegami/work/cpp/llama.cpp/src/llama-batch.h:126:48: error: missing braces around initializer for 'std::__array_traits<int, 1>::_Type' {aka 'int [1]'} [-Werror=missing-braces]
  126 |     std::array<llama_seq_id, 1> seq_id_0 = { 0 }; // default sequence id
      |                                                ^
cc1plus: some warnings being treated as errors
```

The issue is that std::array initialization requires double braces.

## How to fix

This PR changes `{ 0 }` to `{{ 0 }}` for std::array initialization.

This is part of a series of commits to fix missing braces warnings across the codebase.
- src/llama-batch.h <- This PR is here.
- src/llama-context.cpp
- tests/test-backend-ops.cpp
- tests/test-gguf.cpp
- tools/mtmd/clip.cpp

Benefits:
- std::array is a struct containing a C-style array, requiring nested braces
- Enables stricter compiler warnings to catch potential issues

src/llama-batch.h

@@ -123,7 +123,7 @@ private:
     uint32_t n_seq_max;
     uint32_t n_outputs;
 
-    std::array<llama_seq_id, 1> seq_id_0 = { 0 }; // default sequence id
+    std::array<llama_seq_id, 1> seq_id_0 = {{ 0 }}; // default sequence id
 
     std::vector<llama_pos>      pos;
     std::vector<int32_t>        n_seq_id;