ggml : fixed check for _MSC_VER (#4535)

Co-authored-by: Eric Sommerlade <ersomme@microsoft.com>

ggml.h

@@ -303,7 +303,7 @@ extern "C" {
 
 #if defined(__ARM_NEON) && defined(__CUDACC__)
     typedef half ggml_fp16_t;
-#elif defined(__ARM_NEON)
+#elif defined(__ARM_NEON) && !defined(_MSC_VER)
     typedef __fp16 ggml_fp16_t;
 #else
     typedef uint16_t ggml_fp16_t;