Fix ort_genai_c.h compilation in C mode by using conditional includes (microsoft#1516)

The `ort_genai_c.h` header was failing to compile when used as a C file
due to the inclusion of `<cstddef>`, which is a C++-only header.

## Problem
```c
// This would fail when compiling as C
#include "ort_genai_c.h"
```

Error:
```
onnxruntime-genai/src/ort_genai_c.h:7:10: fatal error: 'cstddef' file not found
#include <cstddef>
         ^~~~~~~~~
1 error generated.
```

## Solution
Replaced the unconditional `#include <cstddef>` with conditional
includes:

```c
#ifdef __cplusplus
# include <cstddef>
#else
# include <stddef.h>
# include <stdbool.h>
#endif
```

This ensures:
- **C++ compilation**: Uses `<cstddef>` (provides `size_t`)
- **C compilation**: Uses `<stddef.h>` (provides `size_t`) and
`<stdbool.h>` (provides `bool`)

## Testing
Verified the fix works with both clang and gcc in C and C++ modes:
- ✅ C compilation with clang
- ✅ C compilation with gcc  
- ✅ C++ compilation with clang++
- ✅ C++ compilation with g++

The change is minimal (7 lines added, 1 removed) and preserves all
existing functionality while enabling the header to be used from pure C
code as intended.

Fixes microsoft#1512.

---

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: baijumeswani <[email protected]>
Co-authored-by: Baiju Meswani <[email protected]>

Author: Copilot

src/ort_genai_c.h

@@ -4,7 +4,13 @@
 #pragma once
 
 #include <stdint.h>
+
+#ifdef __cplusplus
 #include <cstddef>
+#else
+#include <stddef.h>
+#include <stdbool.h>
+#endif
 
 #ifdef __cplusplus
 extern "C" {
@@ -689,4 +695,4 @@ OGA_EXPORT OgaResult* OGA_API_CALL OgaSetActiveAdapter(OgaGenerator* generator,
 #ifdef __cplusplus
 }
 #endif
-//! @}
+//! @}