fix compile under MINGW

there was a type conflict around INT32 (in libjpeg).

Patch by nipmarsh
Issue 102: http://code.google.com/p/webp/issues/detail?id=102

Change-Id: I6425b6b9b9861bcaa8bb1adc8a78541e44031a6e
diff --git a/examples/stopwatch.h b/examples/stopwatch.h
index 1a0cf29..de49ffe 100644
--- a/examples/stopwatch.h
+++ b/examples/stopwatch.h
@@ -12,7 +12,7 @@
 #ifndef WEBP_EXAMPLES_STOPWATCH_H_
 #define WEBP_EXAMPLES_STOPWATCH_H_
 
-#ifdef _WIN32
+#if defined _WIN32 && !defined __GNUC__
 #include <windows.h>
 
 typedef LARGE_INTEGER Stopwatch;