Fix broken build w/ Visual C++ < 2010

Regression introduced by dfefba77520ded5c5fd4864e76352a5f3eb23e74
(Windows doesn't always have stdint.h.)
diff --git a/jmemmgr.c b/jmemmgr.c
index b17c5c5..8dfb633 100644
--- a/jmemmgr.c
+++ b/jmemmgr.c
@@ -32,7 +32,9 @@
 #include "jinclude.h"
 #include "jpeglib.h"
 #include "jmemsys.h"            /* import the system-dependent declarations */
+#ifndef _WIN32
 #include <stdint.h>
+#endif
 #include <limits.h>
 
 #ifndef NO_GETENV