build : move _WIN32_WINNT definition to headers (llama/17736)

Previously, cmake was forcing `_WIN32_WINNT=0x0A00` for MinGW builds,
This caused "macro redefined" warnings with toolchains that define the version.

This also removes the `GGML_WIN_VER` variable as it is no longer needed.

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

ggml/include/ggml.h

@@ -204,6 +204,10 @@
 #    define GGML_ATTRIBUTE_FORMAT(...) __attribute__((format(printf, __VA_ARGS__)))
 #endif
 
+#if defined(_WIN32) && !defined(_WIN32_WINNT)
+#    define _WIN32_WINNT 0x0A00
+#endif
+
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>