FileChooser: Fix build compatibility issue with MinGW

The recent IFileChooser update to the JUCE FileChooser meant that
Windows build machines must be running Vista or newer.

For compilers other than MinGW, we were already requiring that the build
machine must be running Windows 8 or newer.

modules/juce_core/native/juce_BasicNativeHeaders.h

@@ -132,7 +132,7 @@
  #define STRICT 1
  #define WIN32_LEAN_AND_MEAN 1
  #if JUCE_MINGW
-  #define _WIN32_WINNT 0x0501
+  #define _WIN32_WINNT 0x0600
  #else
   #define _WIN32_WINNT 0x0602
  #endif