Fix fileno causing compile error when #defined

Error:
expected unqualified-id before '(' token
 int fmt::BufferedFile::fileno() const {

This is an issue with Android NDK and mingw32.

https://github.com/Gachapen/cppformat/commit/c2ffa146841535080a3483471fd9eec77c119c36

test/posix-test.cc

@@ -31,7 +31,7 @@
 #include "posix.h"
 #include "util.h"
 
-#ifdef __MINGW32__
+#ifdef fileno
 # undef fileno
 #endif