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

posix.cc

@@ -55,11 +55,14 @@
 
 # ifdef __MINGW32__
 #  define _SH_DENYNO 0x40
-#  undef fileno
 # endif
 
 #endif  // _WIN32
 
+#ifdef fileno
+# undef fileno
+#endif
+
 namespace {
 #ifdef _WIN32
 // Return type of read and write functions.

test/posix-test.cc

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