ROMReader: fix build error with mingw

../../ROMReader.cpp: In function 'void* STDROMReaderInit(const char*)':
../../ROMReader.cpp:31:36: error: cannot convert 'stat*' to '_stat32*' for argument '2' to 'int _stat32(const char*, _stat32*)'
 #define stat(...) _stat(__VA_ARGS__)
                                    ^

desmume/src/ROMReader.cpp

@@ -79,7 +79,7 @@ struct STDROMReaderData
 
 void* STDROMReaderInit(const char* filename)
 {
-#ifndef _MSC_VER
+#if !defined(_MSC_VER) && !defined(__MINGW32__)
 	struct stat sb;
 	if (stat(filename, &sb) == -1)
 		return 0;