Simplify to include lower-case windows.h always, fix compile on mingw32 (#747)

ggml.c

@@ -26,14 +26,9 @@
 #define static_assert(cond, msg) struct global_scope_noop_trick
 #endif
 
-#if defined _MSC_VER || defined(__MINGW32__)
+#if defined(_WIN32)
 
-#if !defined(__MINGW32__)
-#include <Windows.h>
-#else
-// ref: https://github.com/ggerganov/whisper.cpp/issues/168
 #include <windows.h>
-#endif
 
 typedef volatile LONG atomic_int;
 typedef atomic_int atomic_bool;