Fix ncnn error in MinGW compilation of windows system (#1645)

src/platform.h.in

@@ -25,7 +25,7 @@
 #cmakedefine01 NCNN_REQUANT
 #cmakedefine01 NCNN_AVX2
 
-#ifdef _WIN32
+#if (defined _WIN32 && !(defined __MINGW32__))
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 #include <process.h>
@@ -39,7 +39,7 @@
 
 namespace ncnn {
 
-#ifdef _WIN32
+#if (defined _WIN32 && !(defined __MINGW32__))
 class Mutex
 {
 public:
@@ -75,7 +75,7 @@ private:
     Mutex& mutex;
 };
 
-#if _WIN32
+#if (defined _WIN32 && !(defined __MINGW32__))
 class ConditionVariable
 {
 public:
@@ -101,7 +101,7 @@ private:
 };
 #endif // _WIN32
 
-#if _WIN32
+#if (defined _WIN32 && !(defined __MINGW32__))
 static unsigned __stdcall start_wrapper(void* args);
 class Thread
 {