qt: Fix usage of unconverted path in plat_fopen64

UTF-8 paths are not supported on all Windows installations, only some of them. This was only accounted for in the 32-bit plat_fopen function, not on 64-bit plat_fopen64. Fix that oversight.

src/qt/qt_platform.cpp

@@ -167,7 +167,7 @@ plat_fopen(const char *path, const char *mode)
 FILE *
 plat_fopen64(const char *path, const char *mode)
 {
-    return fopen(path, mode);
+    return fopen(QString::fromUtf8(path).toLocal8Bit(), mode);
 }
 
 int