ggml : suppress unknown pragma 'GCC' on windows (ggerganov#8460)

This commit adds a macro guard to pragma GCC to avoid the following
warning on windows:

```console
C:\llama.cpp\ggml\src\ggml-aarch64.c(17,9): warning C4068:
unknown pragma 'GCC' [C:\lama.cpp\build\ggml\src\ggml.vcxproj]
```

ggml/src/ggml-aarch64.c

@@ -14,7 +14,9 @@
 
 #include "ggml-aarch64.h"
 
+#if defined(__GNUC__)
 #pragma GCC diagnostic ignored "-Woverlength-strings"
+#endif
 
 #define UNUSED GGML_UNUSED