vpxdec: avoid redefining fileno() on mingw

Correct ifdef check for Visual Studio's compiler, mingw builds can use
unistd.h.

Change-Id: I25c5ee46ef75ffbff03f560293dfb3d6bed55f70
diff --git a/vpxdec.c b/vpxdec.c
index 6a1a0f5..7401101 100644
--- a/vpxdec.c
+++ b/vpxdec.c
@@ -32,7 +32,7 @@
 #include "nestegg/include/nestegg/nestegg.h"
 
 #if CONFIG_OS_SUPPORT
-#if defined(_WIN32)
+#if defined(_MSC_VER)
 #include <io.h>
 #define snprintf _snprintf
 #define isatty   _isatty