jinja : add unordered_map include to value.h [no ci] (#19205)

jinja : add unordered_map include to value.h [no ci] (#19205)

On macos Sequoia 15.7.3, x86_64, the build has recently started failing with
```
In file included from .../code/cpp/llama.cpp/common/jinja/string.cpp:2:
.../code/cpp/llama.cpp/common/./jinja/value.h:478:10: error: no template named 'unordered_map' in namespace 'std'
  478 |     std::unordered_map<value, value, value_hasher, value_equivalence> unordered;
      |     ~~~~~^
In file included from .../code/cpp/llama.cpp/common/jinja/caps.cpp:1:
.../code/cpp/llama.cpp/common/jinja/value.h:478:10: error: no template named 'unordered_map' in namespace 'std'
  478 |     std::unordered_map<value, value, value_hasher, value_equivalence> unordered;
      |     ~~~~~^
In file included from .../code/cpp/llama.cpp/common/jinja/value.cpp:1:
In file included from .../code/cpp/llama.cpp/common/jinja/runtime.h:4:
.../code/cpp/llama.cpp/common/jinja/value.h:478:10: error: no template named 'unordered_map' in namespace 'std'
  478 |     std::unordered_map<value, value, value_hasher, value_equivalence> unordered;
[...]
```

After a bit of digging to make sure all the appropriate flags were used, I notifced that the necessary header was not included. This fixes the build for me and should not affect negatively other builds that for some reasons were already succeeding