ggml : fix avx512bf16 build (#18623)

- include `immintrin.h` when required
- remove unused m512bh

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

ggml/src/ggml.c

@@ -53,13 +53,15 @@
 
 #define UNUSED GGML_UNUSED
 
+// Needed for ggml_fp32_to_bf16_row()
+#if defined(__AVX512BF16__)
 #if defined(_MSC_VER)
-#define m512bh(p) p
 #define m512i(p) p
 #else
-#define m512bh(p) (__m512bh)(p)
+#include <immintrin.h>
 #define m512i(p) (__m512i)(p)
-#endif
+#endif // defined(_MSC_VER)
+#endif // defined(__AVX512BF16__)
 
 #if defined(__linux__) || \
     defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || \